通达信板块数据接口-深度学习前馈神经网络自动梯度计算和优化问题

这篇接上一篇深度学习第3章前馈神经网络实验五pytorch实现

3自动梯度计算和预定义算子

虽然我们能够通过模块化的方式比较好地对神经网络进行组装，但是每个模块的梯度计算过程仍然十分繁琐且容易出错。在深度学习框架中，已经封装了自动梯度计算的功能，我们只需要聚焦模型架构，不再需要耗费精力进行计算梯度。

pytorch提供了nn.Module类，来方便快速的实现自己的层和模型。模型和层都可以基于nn.Module扩充实现，模型只是一种特殊的层。

继承了nn.Module类的算子中，可以在内部直接调用其它继承nn.Module类的算子，pytorch框架会自动识别算子中内嵌的nn.Module类算子，并自动计算它们的梯度，并在优化时更新它们的参数。

torch.nn.Module是所有神经网络模块的基类，所有的神经网络模型都应该继承这个基类，即pytorch里面一切自定义操作基本上都是继承nn.Module类来实现的，并进行重载_init_和forward函数。即在_init_进行模型子模块的构建，在forward进行子模块的拼接。每个类都有一个对应的nn.funcational函数，类定义了所需要的arguments和模块的parameters,在forward函数中将arguments和parameters传给nn.funcational的对应的函数来实现forward功能。

注意事项：

一般把网络中具有可学习参数的层放在构造函数__init__中forward方法是必须要重写的，它是实现模型的功能，实现各个层之间的连接关系的核心

1利用预定义算子重新实现前馈神经网络

下面我们使用Pytorch的预定义算子来重新实现二分类任务。主要使用到的预定义算子为torch.nn.Linear

torch.nn.Linear算子可以接受一个形状为[∗,Hin][*,H_{in}][∗,Hin]的输入张量，其中"∗""*""∗"表示张量中可以有任意的其它额外维度，并计算它与形状为[out_features,in_features][out\_features,in\_features][out_features,in_features]的权重矩阵的乘积，然后生成形状为[∗,Hout][*,H_{out}][∗,Hout]的输出张量。torch.nn.Linear算子默认有偏置参数，可以通过bias=False设置不带偏置。代码实现如下：

import torch.nn as nn
import torch.nn.functional as F
from torch.nn.init import constant_, normal_, uniform_
 
 
class Model_MLP_L2_V2(nn.Module):
    def __init__(self, input_size, hidden_size, output_size):
        super(Model_MLP_L2_V2, self).__init__()
        self.fc1 = nn.Linear(input_size, hidden_size)
        normal_(self.fc1.weight, mean=0., std=1.)
        constant_(self.fc1.bias, val=0.0)
        self.fc2 = nn.Linear(hidden_size, output_size)
        normal_(self.fc2.weight, mean=0., std=1.)
        constant_(self.fc2.bias, val=0.0)
        self.act_fn = torch.sigmoid
 
    # 前向计算
    def forward(self, inputs):
        z1 = self.fc1(inputs)
        a1 = self.act_fn(z1)
        z2 = self.fc2(a1)
        a2 = self.act_fn(z2)
        return a2

2完善Runner类

基于上一节实现的RunnerV2_1类，本节的RunnerV2_2类在训练过程中使用自动梯度计算；模型保存时，使用state_dict方法获取模型参数；模型加载时，使用set_state_dict方法加载模型参数.

import torch
 
class RunnerV2_2(object):
    def __init__(self, model, optimizer, metric, loss_fn, **kwargs):
        self.model = model
        self.optimizer = optimizer
        self.loss_fn = loss_fn
        self.metric = metric
 
        # 记录训练过程中的评估指标变化情况
        self.train_scores = []
        self.dev_scores = []
 
        # 记录训练过程中的评价指标变化情况
        self.train_loss = []
        self.dev_loss = []
 
    def train(self, train_set, dev_set, **kwargs):
        # 将模型切换为训练模式
        self.model.train()
 
        # 传入训练轮数，如果没有传入值则默认为0
        num_epochs = kwargs.get("num_epochs", 0)
        # 传入log打印频率，如果没有传入值则默认为100
        log_epochs = kwargs.get("log_epochs", 100)
        # 传入模型保存路径，如果没有传入值则默认为"best_model.pdparams"
        save_path = kwargs.get("save_path", "best_model.pdparams")
 
        # log打印函数，如果没有传入则默认为"None"
        custom_print_log = kwargs.get("custom_print_log", None)
 
        # 记录全局最优指标
        best_score = 0
        # 进行num_epochs轮训练
        for epoch in range(num_epochs):
            X, y = train_set
            # 获取模型预测
            logits = self.model(X)
            # 计算交叉熵损失
            trn_loss = self.loss_fn(logits, y)
            self.train_loss.append(trn_loss.item())
            # 计算评估指标
            trn_score = self.metric(logits, y).item()
            self.train_scores.append(trn_score)
 
            # 自动计算参数梯度
            trn_loss.backward()
            if custom_print_log is not None:
                # 打印每一层的梯度
                custom_print_log(self)
 
            # 参数更新
            self.optimizer.step()
            # 清空梯度
            self.optimizer.zero_grad()
 
            dev_score, dev_loss = self.evaluate(dev_set)
            # 如果当前指标为最优指标，保存该模型
            if dev_score > best_score:
                self.save_model(save_path)
                print(f"[Evaluate] best accuracy performence has been updated: {best_score:.5f} --> {dev_score:.5f}")
                best_score = dev_score
 
            if log_epochs and epoch % log_epochs == 0:
                print(f"[Train] epoch: {epoch}/{num_epochs}, loss: {trn_loss.item()}")
 
    # 模型评估阶段，使用"torch.no_grad()"控制不计算和存储梯度
    @torch.no_grad()
    def evaluate(self, data_set):
        # 将模型切换为评估模式
        self.model.eval()
 
        X, y = data_set
        # 计算模型输出
        logits = self.model(X)
        # 计算损失函数
        loss = self.loss_fn(logits, y).item()
        self.dev_loss.append(loss)
        # 计算评估指标
        score = self.metric(logits, y).item()
        self.dev_scores.append(score)
        return score, loss
 
    def predict(self, X):
        # 将模型切换为评估模式
        self.model.eval()
        return self.model(X)
 
    # 使用"model.state_dict()"获取模型参数，并进行保存
    def save_model(self, saved_path):
        torch.save(self.model.state_dict(), saved_path)
 
    # 使用"model.set_state_dict"加载模型参数
    def load_model(self, model_path):
        state_dict = torch.load(model_path)
        self.model.set_state_dict(state_dict)

3模型训练

实例化RunnerV2类，并传入训练配置，代码实现如下：

from metric import accuracy
 
# 设置模型
input_size = 2
hidden_size = 5
output_size = 1
model = Model_MLP_L2_V2(input_size=input_size, hidden_size=hidden_size, output_size=output_size)
 
# 设置损失函数
loss_fn = F.binary_cross_entropy
 
# 设置优化器
learning_rate = 0.2
optimizer = torch.optim.SGD(model.parameters(), learning_rate)
 
# 设置评价指标
metric = accuracy
 
# 其他参数
epoch_num = 1000
saved_path = "best_model.pdparams"
 
# 实例化RunnerV2类，并传入训练配置
runner = RunnerV2_2(model, optimizer, metric, loss_fn)
runner.train([X_train, y_train], [X_dev, y_dev], num_epochs=epoch_num, log_epochs=50, save_path="best_model.pdparams")

运行结果：

[Evaluate] best accuracy performence has been updated: 0.00000 --> 0.50000
[Train] epoch: 0/1000, loss: 0.8091480135917664
[Evaluate] best accuracy performence has been updated: 0.50000 --> 0.51250
[Evaluate] best accuracy performence has been updated: 0.51250 --> 0.52500
[Evaluate] best accuracy performence has been updated: 0.52500 --> 0.54375
[Evaluate] best accuracy performence has been updated: 0.54375 --> 0.59375
[Evaluate] best accuracy performence has been updated: 0.59375 --> 0.60000
[Evaluate] best accuracy performence has been updated: 0.60000 --> 0.62500
[Evaluate] best accuracy performence has been updated: 0.62500 --> 0.65625
[Evaluate] best accuracy performence has been updated: 0.65625 --> 0.69375
[Evaluate] best accuracy performence has been updated: 0.69375 --> 0.70625
[Evaluate] best accuracy performence has been updated: 0.70625 --> 0.71250
[Evaluate] best accuracy performence has been updated: 0.71250 --> 0.72500
[Evaluate] best accuracy performence has been updated: 0.72500 --> 0.73750
[Train] epoch: 50/1000, loss: 0.5222678184509277
[Evaluate] best accuracy performence has been updated: 0.73750 --> 0.74375
[Train] epoch: 100/1000, loss: 0.48690247535705566
[Evaluate] best accuracy performence has been updated: 0.74375 --> 0.75000
[Evaluate] best accuracy performence has been updated: 0.75000 --> 0.75625
[Train] epoch: 150/1000, loss: 0.4598698616027832
[Train] epoch: 200/1000, loss: 0.4409264028072357
[Evaluate] best accuracy performence has been updated: 0.75625 --> 0.76250
[Evaluate] best accuracy performence has been updated: 0.76250 --> 0.76875
[Train] epoch: 250/1000, loss: 0.4285047650337219
[Evaluate] best accuracy performence has been updated: 0.76875 --> 0.77500
[Train] epoch: 300/1000, loss: 0.4206601083278656
[Train] epoch: 350/1000, loss: 0.4157894551753998
[Train] epoch: 400/1000, loss: 0.41277965903282166
[Train] epoch: 450/1000, loss: 0.4109122157096863
[Train] epoch: 500/1000, loss: 0.4097384512424469
[Train] epoch: 550/1000, loss: 0.4089827537536621
[Train] epoch: 600/1000, loss: 0.4084780216217041
[Train] epoch: 650/1000, loss: 0.40812379121780396
[Train] epoch: 700/1000, loss: 0.4078601896762848
[Train] epoch: 750/1000, loss: 0.4076516032218933
[Train] epoch: 800/1000, loss: 0.40747684240341187
[Train] epoch: 850/1000, loss: 0.40732353925704956
[Train] epoch: 900/1000, loss: 0.40718430280685425
[Train] epoch: 950/1000, loss: 0.4070546627044678

将训练过程中训练集与验证集的准确率变化情况进行可视化。

import matplotlib.pyplot as plt
# 可视化观察训练集与验证集的指标变化情况
def plot(runner, fig_name):
    plt.figure(figsize=(10,5))
    epochs = [i for i in range(len(runner.train_scores))]

    plt.subplot(1,2,1)
    plt.plot(epochs, runner.train_loss, color="#e4007f", label="Train loss")
    plt.plot(epochs, runner.dev_loss, color="#f19ec2", linestyle="--", label="Dev loss")
    # 绘制坐标轴和图例
    plt.ylabel("loss", fontsize="large")
    plt.xlabel("epoch", fontsize="large")
    plt.legend(loc="upper right", fontsize="x-large")

    plt.subplot(1,2,2)
    plt.plot(epochs, runner.train_scores, color="#e4007f", label="Train accuracy")
    plt.plot(epochs, runner.dev_scores, color="#f19ec2", linestyle="--", label="Dev accuracy")
    # 绘制坐标轴和图例
    plt.ylabel("score", fontsize="large")
    plt.xlabel("epoch", fontsize="large")
    plt.legend(loc="lower right", fontsize="x-large")
    
    plt.savefig(fig_name)
    plt.show()

plot(runner, "fw-acc.pdf")

运行结果：

4性能评价

使用测试数据对训练完成后的最优模型进行评价，观察模型在测试集上的准确率以及loss情况。代码如下：

# 模型评价
torch.load("best_model.pdparams")
score, loss = runner.evaluate([X_test, y_test])
print("[Test] score/loss: {:.4f}/{:.4f}".format(score, loss))

运行结果：

[Test] score/loss: 0.7950/0.4897

在上述问题的基础上：增加一个3个神经元的隐藏层，再次实现二分类，并与其做对比。

import torch.nn as nn
import torch.nn.functional as F
from torch.nn.init import constant_, normal_, uniform_
import torch
from metric import accuracy
import matplotlib.pyplot as plt
from dataset import make_moons
 
# 采样1000个样本
n_samples = 1000
X, y = make_moons(n_samples=n_samples, shuffle=True, noise=0.5)
num_train = 640
num_dev = 160
num_test = 200
X_train, y_train = X[:num_train], y[:num_train]
X_dev, y_dev = X[num_train:num_train + num_dev], y[num_train:num_train + num_dev]
X_test, y_test = X[num_train + num_dev:], y[num_train + num_dev:]
y_train = y_train.reshape([-1, 1])
y_dev = y_dev.reshape([-1, 1])
y_test = y_test.reshape([-1, 1])
 
 
class Model_MLP_L2_V4(nn.Module):
    def __init__(self, input_size, hidden_size, hidden_size_3, output_size):
        super(Model_MLP_L2_V4, self).__init__()
        self.fc1 = nn.Linear(input_size, hidden_size)
        normal_(self.fc1.weight, mean=0., std=1.)
        constant_(self.fc1.bias, val=0.0)
        self.fc2 = nn.Linear(hidden_size, hidden_size_3)
        normal_(self.fc2.weight, mean=0., std=1.)
        constant_(self.fc2.bias, val=0.0)
        self.fc3 = nn.Linear(hidden_size_3, output_size)
        normal_(self.fc3.weight, mean=0., std=1.)
        constant_(self.fc3.bias, val=0.0)
        self.act_fn = torch.sigmoid
 
    # 前向计算
    def forward(self, inputs):
        z1 = self.fc1(inputs.to(torch.float32))
        a1 = self.act_fn(z1)
        z2 = self.fc2(a1)
        a2 = self.act_fn(z2)
        z3 = self.fc3(a2)
        a3 = self.act_fn(z3)
        return a3
 
 
class RunnerV2_2(object):
    def __init__(self, model, optimizer, metric, loss_fn, **kwargs):
        self.model = model
        self.optimizer = optimizer
        self.loss_fn = loss_fn
        self.metric = metric
 
        # 记录训练过程中的评估指标变化情况
        self.train_scores = []
        self.dev_scores = []
 
        # 记录训练过程中的评价指标变化情况
        self.train_loss = []
        self.dev_loss = []
 
    def train(self, train_set, dev_set, **kwargs):
        # 将模型切换为训练模式
        self.model.train()
 
        # 传入训练轮数，如果没有传入值则默认为0
        num_epochs = kwargs.get("num_epochs", 0)
        # 传入log打印频率，如果没有传入值则默认为100
        log_epochs = kwargs.get("log_epochs", 100)
        # 传入模型保存路径，如果没有传入值则默认为"best_model.pdparams"
        save_path = kwargs.get("save_path", "best_model.pdparams")
 
        # log打印函数，如果没有传入则默认为"None"
        custom_print_log = kwargs.get("custom_print_log", None)
 
        # 记录全局最优指标
        best_score = 0
        # 进行num_epochs轮训练
        for epoch in range(num_epochs):
            X, y = train_set
            # 获取模型预测
            logits = self.model(X)
            # 计算交叉熵损失
            trn_loss = self.loss_fn(logits, y)
            self.train_loss.append(trn_loss.item())
            # 计算评估指标
            trn_score = self.metric(logits, y).item()
            self.train_scores.append(trn_score)
 
            # 自动计算参数梯度
            trn_loss.backward()
            if custom_print_log is not None:
                # 打印每一层的梯度
                custom_print_log(self)
 
            # 参数更新
            self.optimizer.step()
            # 清空梯度
            self.optimizer.zero_grad()
 
            dev_score, dev_loss = self.evaluate(dev_set)
            # 如果当前指标为最优指标，保存该模型
            if dev_score > best_score:
                self.save_model(save_path)
                print(f"[Evaluate] best accuracy performence has been updated: {best_score:.5f} --> {dev_score:.5f}")
                best_score = dev_score
 
            if log_epochs and epoch % log_epochs == 0:
                print(f"[Train] epoch: {epoch}/{num_epochs}, loss: {trn_loss.item()}")
 
    # 模型评估阶段，使用"torch.no_grad()"控制不计算和存储梯度
    @torch.no_grad()
    def evaluate(self, data_set):
        # 将模型切换为评估模式
        self.model.eval()
 
        X, y = data_set
        # 计算模型输出
        logits = self.model(X)
        # 计算损失函数
        loss = self.loss_fn(logits, y).item()
        self.dev_loss.append(loss)
        # 计算评估指标
        score = self.metric(logits, y).item()
        self.dev_scores.append(score)
        return score, loss
 
    def predict(self, X):
        # 将模型切换为评估模式
        self.model.eval()
        return self.model(X)
 
    # 使用"model.state_dict()"获取模型参数，并进行保存
    def save_model(self, saved_path):
        torch.save(self.model.state_dict(), saved_path)
 
    # 使用"model.set_state_dict"加载模型参数
    def load_model(self, model_path):
        state_dict = torch.load(model_path)
        self.model.set_state_dict(state_dict)
 
 
# 设置模型
input_size = 2
hidden_size = 5
hidden_size_3 = 3
output_size = 1
model = Model_MLP_L2_V4(input_size=input_size, hidden_size=hidden_size, hidden_size_3=3, output_size=output_size)
 
# 设置损失函数
loss_fn = F.binary_cross_entropy
 
# 设置优化器
learning_rate = 0.2
optimizer = torch.optim.SGD(model.parameters(), learning_rate)
 
# 设置评价指标
metric = accuracy
 
# 其他参数
epoch_num = 1000
saved_path = "best_model.pdparams"
 
# 实例化RunnerV2类，并传入训练配置
runner = RunnerV2_2(model, optimizer, metric, loss_fn)
runner.train([X_train, y_train], [X_dev, y_dev], num_epochs=epoch_num, log_epochs=50, save_path="best_model.pdparams")
 
 
# 可视化观察训练集与验证集的指标变化情况
def plot(runner, fig_name):
    plt.figure(figsize=(10, 5))
    epochs = [i for i in range(len(runner.train_scores))]
 
    plt.subplot(1, 2, 1)
    plt.plot(epochs, runner.train_loss, color="#e4007f", label="Train loss")
    plt.plot(epochs, runner.dev_loss, color="#f19ec2", linestyle="--", label="Dev loss")
    # 绘制坐标轴和图例
    plt.ylabel("loss", fontsize="large")
    plt.xlabel("epoch", fontsize="large")
    plt.legend(loc="upper right", fontsize="x-large")
 
    plt.subplot(1, 2, 2)
    plt.plot(epochs, runner.train_scores, color="#e4007f", label="Train accuracy")
    plt.plot(epochs, runner.dev_scores, color="#f19ec2", linestyle="--", label="Dev accuracy")
    # 绘制坐标轴和图例
    plt.ylabel("score", fontsize="large")
    plt.xlabel("epoch", fontsize="large")
    plt.legend(loc="lower right", fontsize="x-large")
 
    plt.savefig(fig_name)
    plt.show()
 
 
plot(runner, "fw-acc.pdf")
 
# 模型评价
torch.load("best_model.pdparams")
score, loss = runner.evaluate([X_test, y_test])
print("[Test] score/loss: {:.4f}/{:.4f}".format(score, loss))

运行结果：

outer_circ_x.shape: torch.Size([500]) outer_circ_y.shape: torch.Size([500])
outer_circ_x.shape: torch.Size([500]) inner_circ_y.shape: torch.Size([500])
after concat shape: torch.Size([1000])
X shape: torch.Size([1000, 2])
y shape: torch.Size([1000])
[Evaluate] best accuracy performence has been updated: 0.00000 --> 0.45000
[Train] epoch: 0/1000, loss: 0.6945466995239258
[Evaluate] best accuracy performence has been updated: 0.45000 --> 0.48750
[Evaluate] best accuracy performence has been updated: 0.48750 --> 0.54375
[Evaluate] best accuracy performence has been updated: 0.54375 --> 0.59375
[Evaluate] best accuracy performence has been updated: 0.59375 --> 0.63750
[Evaluate] best accuracy performence has been updated: 0.63750 --> 0.66250
[Evaluate] best accuracy performence has been updated: 0.66250 --> 0.69375
[Evaluate] best accuracy performence has been updated: 0.69375 --> 0.73125
[Evaluate] best accuracy performence has been updated: 0.73125 --> 0.75000
[Evaluate] best accuracy performence has been updated: 0.75000 --> 0.76875
[Evaluate] best accuracy performence has been updated: 0.76875 --> 0.78125
[Evaluate] best accuracy performence has been updated: 0.78125 --> 0.80000
[Evaluate] best accuracy performence has been updated: 0.80000 --> 0.81250
[Evaluate] best accuracy performence has been updated: 0.81250 --> 0.81875
[Train] epoch: 50/1000, loss: 0.6421956419944763
[Train] epoch: 100/1000, loss: 0.5900207757949829
[Train] epoch: 150/1000, loss: 0.5342556238174438
[Train] epoch: 200/1000, loss: 0.4871096611022949
[Train] epoch: 250/1000, loss: 0.45485734939575195
[Train] epoch: 300/1000, loss: 0.43542352318763733
[Train] epoch: 350/1000, loss: 0.4243297576904297
[Train] epoch: 400/1000, loss: 0.4180295467376709
[Train] epoch: 450/1000, loss: 0.41435056924819946
[Train] epoch: 500/1000, loss: 0.41209086775779724
[Train] epoch: 550/1000, loss: 0.4106135964393616
[Train] epoch: 600/1000, loss: 0.40958476066589355
[Train] epoch: 650/1000, loss: 0.40882715582847595
[Train] epoch: 700/1000, loss: 0.4082435965538025
[Train] epoch: 750/1000, loss: 0.40777772665023804
[Train] epoch: 800/1000, loss: 0.4073949456214905
[Train] epoch: 850/1000, loss: 0.4070724546909332
[Train] epoch: 900/1000, loss: 0.4067947268486023
[Train] epoch: 950/1000, loss: 0.4065505862236023
[Test] score/loss: 0.8150/0.4290

通过对比，我们发现增加隐藏层，可以提高准确率

4优化问题

在本节中，我们通过实践来发现神经网络模型的优化问题，并思考如何改进。

1参数初始化

实现一个神经网络前，需要先初始化模型参数。如果对每一层的权重和偏置都用0初始化，那么通过第一遍前向计算，所有隐藏层神经元的激活值都相同；在反向传播时，所有权重的更新也都相同，这样会导致隐藏层神经元没有差异性，出现对称权重现象。

接下来，将模型参数全都初始化为0，看实验结果。这里重新定义了一个类，两个线性层的参数全都初始化为0。

import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.nn.init import constant_, normal_, uniform_
 
 
class Model_MLP_L2_V4(nn.Module):
    def __init__(self, input_size, hidden_size, output_size):
        super(Model_MLP_L2_V4, self).__init__()
        self.fc1 = nn.Linear(input_size, hidden_size)
        constant_(self.fc1.weight, val=0.0)
        constant_(self.fc1.bias, val=0.0)
        self.fc2 = nn.Linear(hidden_size, output_size)
        constant_(self.fc2.weight, val=0.0)
        constant_(self.fc2.bias, val=0.0)
        self.act_fn = torch.sigmoid
 
    # 前向计算
    def forward(self, inputs):
        z1 = self.fc1(inputs)
        a1 = self.act_fn(z1)
        z2 = self.fc2(a1)
        a2 = self.act_fn(z2)
        return a2

def print_weights(runner):
    print("The weights of the Layers：")
    for item in runner.model.named_parameters():
        print(item)

利用Runner类训练模型：

# 设置模型
input_size = 2
hidden_size = 5
output_size = 1
model = Model_MLP_L2_V4(input_size=input_size, hidden_size=hidden_size, output_size=output_size)
 
# 设置损失函数
loss_fn = F.binary_cross_entropy
 
# 设置优化器
learning_rate = 0.2  # 5e-2
optimizer = torch.optim.SGD(model.parameters(), learning_rate)
 
# 设置评价指标
metric = accuracy
 
# 其他参数
epoch = 2000
saved_path = "best_model.pdparams"
 
# 实例化RunnerV2类，并传入训练配置
runner = RunnerV2_2(model, optimizer, metric, loss_fn)
runner.train([X_train, y_train], [X_dev, y_dev], num_epochs=5, log_epochs=50, save_path="best_model.pdparams", custom_print_log=print_weights)

运行结果：

The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[0., 0.],
        [0., 0.],
        [0., 0.],
        [0., 0.],
        [0., 0.]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([0., 0., 0., 0., 0.], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[0., 0., 0., 0., 0.]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([0.], requires_grad=True))
[Evaluate] best accuracy performence has been updated: 0.00000 --> 0.50000
[Train] epoch: 0/5, loss: 0.6931472420692444
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[0., 0.],
        [0., 0.],
        [0., 0.],
        [0., 0.],
        [0., 0.]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([0., 0., 0., 0., 0.], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[0., 0., 0., 0., 0.]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([2.7940e-10], requires_grad=True))
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[0., 0.],
        [0., 0.],
        [0., 0.],
        [0., 0.],
        [0., 0.]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([0., 0., 0., 0., 0.], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[0., 0., 0., 0., 0.]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([5.5879e-10], requires_grad=True))
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[0., 0.],
        [0., 0.],
        [0., 0.],
        [0., 0.],
        [0., 0.]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([0., 0., 0., 0., 0.], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[0., 0., 0., 0., 0.]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([8.3819e-10], requires_grad=True))
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[0., 0.],
        [0., 0.],
        [0., 0.],
        [0., 0.],
        [0., 0.]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([0., 0., 0., 0., 0.], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[0., 0., 0., 0., 0.]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([1.1176e-09], requires_grad=True))

这里我把训练次数提升到5000，运行结果：

# 这里只输出最后五次的结果
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[-0.7841,  1.3522],
        [-0.7841,  1.3522],
        [-0.7841,  1.3522],
        [-0.7841,  1.3522],
        [-0.7841,  1.3522]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([-0.2695, -0.2695, -0.2695, -0.2695, -0.2695], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[-1.6042, -1.6042, -1.6042, -1.6042, -1.6042]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([3.4718], requires_grad=True))
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[-0.7841,  1.3522],
        [-0.7841,  1.3522],
        [-0.7841,  1.3522],
        [-0.7841,  1.3522],
        [-0.7841,  1.3522]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([-0.2695, -0.2695, -0.2695, -0.2695, -0.2695], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[-1.6043, -1.6043, -1.6043, -1.6043, -1.6043]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([3.4719], requires_grad=True))
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[-0.7840,  1.3521],
        [-0.7840,  1.3521],
        [-0.7840,  1.3521],
        [-0.7840,  1.3521],
        [-0.7840,  1.3521]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([-0.2695, -0.2695, -0.2695, -0.2695, -0.2695], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[-1.6043, -1.6043, -1.6043, -1.6043, -1.6043]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([3.4721], requires_grad=True))
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[-0.7840,  1.3521],
        [-0.7840,  1.3521],
        [-0.7840,  1.3521],
        [-0.7840,  1.3521],
        [-0.7840,  1.3521]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([-0.2695, -0.2695, -0.2695, -0.2695, -0.2695], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[-1.6044, -1.6044, -1.6044, -1.6044, -1.6044]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([3.4722], requires_grad=True))
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[-0.7840,  1.3520],
        [-0.7840,  1.3520],
        [-0.7840,  1.3520],
        [-0.7840,  1.3520],
        [-0.7840,  1.3520]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([-0.2695, -0.2695, -0.2695, -0.2695, -0.2695], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[-1.6044, -1.6044, -1.6044, -1.6044, -1.6044]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([3.4723], requires_grad=True))

从输出结果看，前2000次训练，w的值一直是0，也可能改变了但是性能没有改变，这里我们分析一下：这里的w一直在局部极小点徘徊，当到了2000左右，跳到了全局极小点，然后性能进行了提高！！！

我们可以提高学习率来加快www的跳出

这里我把学习率改成训练轮数改为1000：运行结果：

The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[-0.5812,  1.0367],
        [-0.5812,  1.0367],
        [-0.5812,  1.0367],
        [-0.5812,  1.0367],
        [-0.5812,  1.0367]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([-0.3856, -0.3856, -0.3856, -0.3856, -0.3856], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[-2.0969, -2.0969, -2.0969, -2.0969, -2.0969]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([4.0437], requires_grad=True))
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[-0.6149,  1.0128],
        [-0.6149,  1.0128],
        [-0.6149,  1.0128],
        [-0.6149,  1.0128],
        [-0.6149,  1.0128]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([-0.4510, -0.4510, -0.4510, -0.4510, -0.4510], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[-2.0390, -2.0390, -2.0390, -2.0390, -2.0390]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([4.1869], requires_grad=True))
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[-0.5812,  1.0367],
        [-0.5812,  1.0367],
        [-0.5812,  1.0367],
        [-0.5812,  1.0367],
        [-0.5812,  1.0367]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([-0.3856, -0.3856, -0.3856, -0.3856, -0.3856], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[-2.0970, -2.0970, -2.0970, -2.0970, -2.0970]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([4.0437], requires_grad=True))
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[-0.6149,  1.0128],
        [-0.6149,  1.0128],
        [-0.6149,  1.0128],
        [-0.6149,  1.0128],
        [-0.6149,  1.0128]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([-0.4510, -0.4510, -0.4510, -0.4510, -0.4510], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[-2.0391, -2.0391, -2.0391, -2.0391, -2.0391]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([4.1868], requires_grad=True))
The weights of the Layers：
("fc1.weight", Parameter containing:
tensor([[-0.5812,  1.0367],
        [-0.5812,  1.0367],
        [-0.5812,  1.0367],
        [-0.5812,  1.0367],
        [-0.5812,  1.0367]], requires_grad=True))
("fc1.bias", Parameter containing:
tensor([-0.3857, -0.3857, -0.3857, -0.3857, -0.3857], requires_grad=True))
("fc2.weight", Parameter containing:
tensor([[-2.0970, -2.0970, -2.0970, -2.0970, -2.0970]], requires_grad=True))
("fc2.bias", Parameter containing:
tensor([4.0437], requires_grad=True))

这里很快就跳出来了

从输出结果看，二分类准确率为50%左右，说明模型没有学到任何内容。训练和验证loss几乎没有怎么下降。

为了避免对称权重现象，可以使用高斯分布或均匀分布初始化神经网络的参数。

gausian_weights = torch.normal(mean=0.0, std=1.0, size=[10000])
uniform_weights = torch.Tensor(10000)
uniform_weights.uniform_(-1, 1)
# 绘制两种参数分布
plt.figure()
plt.subplot(1, 2, 1)
plt.title("Gausian Distribution")
plt.hist(gausian_weights, bins=200, density=True, color="#f19ec2")
plt.subplot(1, 2, 2)
plt.title("Uniform Distribution")
plt.hist(uniform_weights, bins=200, density=True, color="#e4007f")
plt.savefig("fw-gausian-uniform.pdf")
plt.show()

运行结果：

2梯度消失问题

在神经网络的构建过程中，随着网络层数的增加，理论上网络的拟合能力也应该是越来越好的。但是随着网络变深，参数学习更加困难，容易出现梯度消失问题。

由于Sigmoid型函数的饱和性，饱和区的导数更接近于0，误差经过每一层传递都会不断衰减。当网络层数很深时，梯度就会不停衰减，甚至消失，使得整个网络很难训练，这就是所谓的梯度消失问题。在深度神经网络中，减轻梯度消失问题的方法有很多种，一种简单有效的方式就是使用导数比较大的激活函数，如：ReLU。

下面通过一个简单的实验观察前馈神经网络的梯度消失现象和改进方法。

1模型构建

定义一个前馈神经网络，包含4个隐藏层和1个输出层，通过传入的参数指定激活函数。代码实现如下：

a = torch.Tensor(10000)
 
 
class Model_MLP_L5(nn.Module):
    def __init__(self, input_size, output_size, act="sigmoid", w_init=normal_(a, mean=0.0, std=0.01), b_init=constant_(a, val=1.0)):
        super(Model_MLP_L5, self).__init__()
        self.fc1 = torch.nn.Linear(input_size, 3)
        self.fc2 = torch.nn.Linear(3, 3)
        self.fc3 = torch.nn.Linear(3, 3)
        self.fc4 = torch.nn.Linear(3, 3)
        self.fc5 = torch.nn.Linear(3, output_size)
        # 定义网络使用的激活函数
        if act == "sigmoid":
            self.act = torch.sigmoid
        elif act == "relu":
            self.act = F.relu
        elif act == "lrelu":
            self.act = F.leaky_relu
        else:
            raise ValueError("Please enter sigmoid relu or lrelu!")
        # 初始化线性层权重和偏置参数
        self.init_weights(w_init, b_init)
 
    # 初始化线性层权重和偏置参数
    def init_weights(self, w_init, b_init):
        # 使用"named_sublayers"遍历所有网络层
        for n, m in self.named_parameters():
            # 如果是线性层，则使用指定方式进行参数初始化
            if isinstance(m, nn.Linear):
                w_init(m.weight)
                b_init(m.bias)
 
    def forward(self, inputs):
        outputs = self.fc1(inputs)
        outputs = self.act(outputs)
        outputs = self.fc2(outputs)
        outputs = self.act(outputs)
        outputs = self.fc3(outputs)
        outputs = self.act(outputs)
        outputs = self.fc4(outputs)
        outputs = self.act(outputs)
        outputs = self.fc5(outputs)
        outputs = torch.sigmoid(outputs)
        return outputs

2使用Sigmoid型函数进行训练

使用Sigmoid型函数作为激活函数，为了便于观察梯度消失现象，只进行一轮网络优化。代码实现如下：

定义梯度打印函数

def print_grads(runner):
    # 打印每一层的权重的模
    print("The gradient of the Layers：")
    for name, item in runner.model.named_parameters():
        if len(item.size()) == 2:
             print(name, torch.norm(input=item, p=2))

# 学习率大小
lr = 0.01
# 定义网络，激活函数使用sigmoid
model = Model_MLP_L5(input_size=2, output_size=1, act="sigmoid")
# 定义优化器
optimizer = torch.optim.SGD(lr=lr, params=model.parameters())
# 定义损失函数，使用交叉熵损失函数
loss_fn = F.binary_cross_entropy
# 定义评价指标
metric = accuracy
# 指定梯度打印函数
custom_print_log=print_grads
# 实例化Runner类
runner = RunnerV2_2(model, optimizer, metric, loss_fn)
# 启动训练
runner.train([X_train, y_train], [X_dev, y_dev], num_epochs=1, log_epochs=None, save_path="best_model.pdparams", custom_print_log=custom_print_log)

实例化RunnerV2_2类，并传入训练配置。代码实现如下：

# 实例化Runner类
runner = RunnerV2_2(model, optimizer, metric, loss_fn)

模型训练，打印网络每层梯度值的ℓ2ℓ2ℓ2范数。代码实现如下：

# 启动训练
runner.train([X_train, y_train], [X_dev, y_dev], num_epochs=1, log_epochs=None, save_path="best_model.pdparams", custom_print_log=custom_print_log)

运行结果：

The gradient of the Layers：
fc1.weight tensor(1.0447, grad_fn=<NormBackward1>)
fc2.weight tensor(1.2803, grad_fn=<NormBackward1>)
fc3.weight tensor(0.8694, grad_fn=<NormBackward1>)
fc4.weight tensor(1.0071, grad_fn=<NormBackward1>)
fc5.weight tensor(0.5389, grad_fn=<NormBackward1>)
[Evaluate] best accuracy performence has been updated: 0.00000 --> 0.50000

下展示了使用不同激活函数时，网络每层梯度值的ℓ2范数情况。从结果可以看到，5层的全连接前馈神经网络使用Sigmoid型函数作为激活函数时，梯度经过每一个神经层的传递都会不断衰减，最终传递到第一个神经层时，梯度几乎完全消失。改为ReLU激活函数后，梯度消失现象得到了缓解，每一层的参数都具有梯度值。

3死亡ReLU问题

ReLU激活函数可以一定程度上改善梯度消失问题，但是ReLU函数在某些情况下容易出现死亡ReLU问题，使得网络难以训练。这是由于当x<0时，ReLU函数的输出恒为0。在训练过程中，如果参数在一次不恰当的更新后，某个ReLU神经元在所有训练数据上都不能被激活，那么这个神经元自身参数的梯度永远都会是0，在以后的训练过程中永远都不能被激活。而一种简单有效的优化方式就是将激活函数更换为LeakyReLU、ELU等ReLU的变种。

1使用ReLU进行模型训练

使用第2节中定义的多层全连接前馈网络进行实验，使用ReLU作为激活函数，观察死亡ReLU现象和优化方法。当神经层的偏置被初始化为一个相对于权重较大的负值时，可以想像，输入经过神经层的处理，最终的输出会为负值，从而导致死亡ReLU现象。

# 定义网络，并使用较大的负值来初始化偏置
model = Model_MLP_L5(input_size=2, output_size=1, act="relu", b_init=constant_(a, val=-8.0))

实例化RunnerV2类，启动模型训练，打印网络每层梯度值的ℓ2ℓ2ℓ2范数。代码实现如下：

# 实例化Runner类
runner = RunnerV2_2(model, optimizer, metric, loss_fn)
# 启动训练
runner.train([X_train, y_train], [X_dev, y_dev], num_epochs=1, log_epochs=0, save_path="best_model.pdparams", custom_print_log=custom_print_log)

运行结果：

The gradient of the Layers：
fc1.weight tensor(0.8176, grad_fn=<NormBackward1>)
fc2.weight tensor(0.9802, grad_fn=<NormBackward1>)
fc3.weight tensor(0.9874, grad_fn=<NormBackward1>)
fc4.weight tensor(1.0451, grad_fn=<NormBackward1>)
fc5.weight tensor(0.4850, grad_fn=<NormBackward1>)
[Evaluate] best accuracy performence has been updated: 0.00000 --> 0.50000

从输出结果可以发现，使用ReLU作为激活函数，当满足条件时，会发生死亡ReLU问题，网络训练过程中ReLU神经元的梯度始终为0，参数无法更新。

针对死亡ReLU问题，一种简单有效的优化方式就是将激活函数更换为LeakyReLU、ELU等ReLU的变种。接下来，观察将激活函数更换为LeakyReLU时的梯度情况。

2使用LeakyReLU进行模型训练

将激活函数更换为LeakyReLU进行模型训练，观察梯度情况。代码实现如下：

# 重新定义网络，使用Leaky ReLU激活函数
model = Model_MLP_L5(input_size=2, output_size=1, act="lrelu", b_init=constant_(a, val=-8.0))
 
# 实例化Runner类
runner = RunnerV2_2(model, optimizer, metric, loss_fn)
 
# 启动训练
runner.train([X_train, y_train], [X_dev, y_dev], num_epochs=1, log_epochps=None, save_path="best_model.pdparams", custom_print_log=custom_print_log)

运行结果：

The gradient of the Layers：
fc1.weight tensor(0.7638, grad_fn=<NormBackward1>)
fc2.weight tensor(1.1522, grad_fn=<NormBackward1>)
fc3.weight tensor(1.0465, grad_fn=<NormBackward1>)
fc4.weight tensor(1.0655, grad_fn=<NormBackward1>)
fc5.weight tensor(0.5679, grad_fn=<NormBackward1>)
[Evaluate] best accuracy performence has been updated: 0.00000 --> 0.50000
[Train] epoch: 0/1, loss: 0.7000465989112854

从输出结果可以看到，将激活函数更换为LeakyReLU后，死亡ReLU问题得到了改善，梯度恢复正常，参数也可以正常更新。但是由于LeakyReLU中，x<0时的斜率默认只有0.0所以反向传播时，随着网络层数的加深，梯度值越来越小。如果想要改善这一现象，将LeakyReLU中，x<0时的斜率调大即可。

使用pytorch的预定义算子来重新实现二分类任务。

代码：

import torch.nn as nn
import torch.nn.functional as F
import os
import torch
from abc import abstractmethod
import math
import numpy as np


n_samples = 1000
X, y = make_moons(n_samples=n_samples, shuffle=True, noise=0.15)

num_train = 640
num_dev = 160
num_test = 200

X_train, y_train = X[:num_train], y[:num_train]
X_dev, y_dev = X[num_train:num_train + num_dev], y[num_train:num_train + num_dev]
X_test, y_test = X[num_train + num_dev:], y[num_train + num_dev:]

y_train = y_train.reshape([-1,1])
y_dev = y_dev.reshape([-1,1])
y_test = y_test.reshape([-1,1])

class Model_MLP_L2_V4(torch.nn.Module):
    def __init__(self, input_size, hidden_size, output_size):
        super(Model_MLP_L2_V4, self).__init__()
        self.fc1 = nn.Linear(input_size, hidden_size)
        w=torch.normal(0,0.1,size=(hidden_size,input_size),requires_grad=True)
        self.fc1.weight = nn.Parameter(w)

        self.fc2 = nn.Linear(hidden_size, output_size)
        w = torch.normal(0, 0.1, size=(output_size, hidden_size), requires_grad=True)
        self.fc2.weight = nn.Parameter(w)

        # 使用"torch.nn.functional.sigmoid"定义 Logistic 激活函数
        self.act_fn = torch.sigmoid

    # 前向计算
    def forward(self, inputs):
        z1 = self.fc1(inputs.to(torch.float32))
        a1 = self.act_fn(z1)
        z2 = self.fc2(a1)
        a2 = self.act_fn(z2)
        return a2


# def print_weights(runner):
#     print("The weights of the Layers：")
#
#     for item in runner.model.sublayers():
#         print(item.full_name()
#         for param in item.parameters():
#             print(param.numpy())


class RunnerV2_2(object):
    def __init__(self, model, optimizer, metric, loss_fn, **kwargs):
        self.model = model
        self.optimizer = optimizer
        self.loss_fn = loss_fn
        self.metric = metric

        # 记录训练过程中的评估指标变化情况
        self.train_scores = []
        self.dev_scores = []

        # 记录训练过程中的评价指标变化情况
        self.train_loss = []
        self.dev_loss = []

    def train(self, train_set, dev_set, **kwargs):
        # 将模型切换为训练模式
        self.model.train()

        # 传入训练轮数，如果没有传入值则默认为0
        num_epochs = kwargs.get("num_epochs", 0)
        # 传入log打印频率，如果没有传入值则默认为100
        log_epochs = kwargs.get("log_epochs", 100)
        # 传入模型保存路径，如果没有传入值则默认为"best_model.pdparams"
        save_path = kwargs.get("save_path", "best_model.pdparams")

        # log打印函数，如果没有传入则默认为"None"
        custom_print_log = kwargs.get("custom_print_log", None)

        # 记录全局最优指标
        best_score = 0
        # 进行num_epochs轮训练
        for epoch in range(num_epochs):
            X, y = train_set

            # 获取模型预测
            logits = self.model(X.to(torch.float32))
            # 计算交叉熵损失
            trn_loss = self.loss_fn(logits, y)
            self.train_loss.append(trn_loss.item())
            # 计算评估指标
            trn_score = self.metric(logits, y).item()
            self.train_scores.append(trn_score)

            # 自动计算参数梯度
            trn_loss.backward()
            if custom_print_log is not None:
                # 打印每一层的梯度
                custom_print_log(self)

            # 参数更新
            self.optimizer.step()
            # 清空梯度
            self.optimizer.zero_grad()   # reset gradient

            dev_score, dev_loss = self.evaluate(dev_set)
            # 如果当前指标为最优指标，保存该模型
            if dev_score > best_score:
                self.save_model(save_path)
                print(f"[Evaluate] best accuracy performence has been updated: {best_score:.5f} --> {dev_score:.5f}")
                best_score = dev_score

            if log_epochs and epoch % log_epochs == 0:
                print(f"[Train] epoch: {epoch}/{num_epochs}, loss: {trn_loss.item()}")
    @torch.no_grad()
    def evaluate(self, data_set):
        # 将模型切换为评估模式
        self.model.eval()

        X, y = data_set
        # 计算模型输出
        logits = self.model(X)
        # 计算损失函数
        loss = self.loss_fn(logits, y).item()
        self.dev_loss.append(loss)
        # 计算评估指标
        score = self.metric(logits, y).item()
        self.dev_scores.append(score)
        return score, loss

    # 模型测试阶段，使用"torch.no_grad()"控制不计算和存储梯度
    @torch.no_grad()
    def predict(self, X):
        # 将模型切换为评估模式
        self.model.eval()
        return self.model(X)

    # 使用"model.state_dict()"获取模型参数，并进行保存
    def save_model(self, saved_path):
        torch.save(self.model.state_dict(), saved_path)

    # 使用"model.set_state_dict"加载模型参数
    def load_model(self, model_path):
        state_dict = torch.load(model_path)
        self.model.load_state_dict(state_dict)


# 设置模型
input_size = 2
hidden_size = 5
output_size = 1
model = Model_MLP_L2_V4(input_size=input_size, hidden_size=hidden_size, output_size=output_size)

# 设置损失函数
loss_fn = F.binary_cross_entropy

# 设置优化器
learning_rate = 0.2 #5e-2
optimizer = torch.optim.SGD(model.parameters(),lr=learning_rate)

# 设置评价指标
metric = accuracy

# 其他参数
epoch = 2000
saved_path = "best_model.pdparams"

# 实例化RunnerV2类，并传入训练配置
runner = RunnerV2_2(model, optimizer, metric, loss_fn)

runner.train([X_train, y_train], [X_dev, y_dev], num_epochs = epoch, log_epochs=50, save_path="best_model.pdparams")

plot(runner, "fw-acc.pdf")

#模型评价
runner.load_model("best_model.pdparams")
score, loss = runner.evaluate([X_test, y_test])
print("[Test] score/loss: {:.4f}/{:.4f}".format(score, loss))

这里补充几个函数：

make_moons函数代码：

import torch
# 新增make_moons函数
def make_moons(n_samples=1000, shuffle=True, noise=None):
    n_samples_out = n_samples // 2
    n_samples_in = n_samples - n_samples_out
    outer_circ_x = torch.cos(torch.linspace(0, math.pi, n_samples_out))
    outer_circ_y = torch.sin(torch.linspace(0, math.pi, n_samples_out))
    inner_circ_x = 1 - torch.cos(torch.linspace(0, math.pi, n_samples_in))
    inner_circ_y = 0.5 - torch.sin(torch.linspace(0, math.pi, n_samples_in))
    X = torch.stack(
        [torch.cat([outer_circ_x, inner_circ_x]),
         torch.cat([outer_circ_y, inner_circ_y])],
         axis=1
    )
    y = torch.cat(
        [torch.zeros([n_samples_out]), torch.ones([n_samples_in])]
    )
    if shuffle:
        idx = torch.randperm(X.shape[0])
        X = X[idx]
        y = y[idx]
    if noise is not None:
        X += np.random.normal(0.0, noise, X.shape)

    return X, y

accuracy函数代码：

def accuracy(preds, labels):
    # 判断是二分类任务还是多分类任务，preds.shape[1]=1时为二分类任务，preds.shape[1]>1时为多分类任务
    if preds.shape[1] == 1:
        preds=(preds>=0.5).to(torch.float32)

    else:
        preds = torch.argmax(preds,dim=1).int()

    return torch.mean((preds == labels).float())

plot函数代码：

import matplotlib.pyplot as plt
def plot(runner, fig_name):
    plt.figure(figsize=(10, 5))
    epochs = [i for i in range(len(runner.train_scores))]

    plt.subplot(1, 2, 1)
    plt.plot(epochs, runner.train_loss, color="#e4007f", label="Train loss")
    plt.plot(epochs, runner.dev_loss, color="#f19ec2", linestyle="--", label="Dev loss")
    # 绘制坐标轴和图例
    plt.ylabel("loss", fontsize="large")
    plt.xlabel("epoch", fontsize="large")
    plt.legend(loc="upper right", fontsize="x-large")

    plt.subplot(1, 2, 2)
    plt.plot(epochs, runner.train_scores, color="#e4007f", label="Train accuracy")
    plt.plot(epochs, runner.dev_scores, color="#f19ec2", linestyle="--", label="Dev accuracy")
    # 绘制坐标轴和图例
    plt.ylabel("score", fontsize="large")
    plt.xlabel("epoch", fontsize="large")
    plt.legend(loc="lower right", fontsize="x-large")
    plt.savefig(fig_name)
    plt.show()

运行结果：

[Evaluate] best accuracy performence has been updated: 0.00000 --> 0.46250
[Train] epoch: 0/2000, loss: 0.7185732126235962
[Train] epoch: 50/2000, loss: 0.6900826096534729
[Evaluate] best accuracy performence has been updated: 0.46250 --> 0.46875
[Evaluate] best accuracy performence has been updated: 0.46875 --> 0.47500
[Evaluate] best accuracy performence has been updated: 0.47500 --> 0.49375
[Evaluate] best accuracy performence has been updated: 0.49375 --> 0.50625
[Evaluate] best accuracy performence has been updated: 0.50625 --> 0.52500
[Evaluate] best accuracy performence has been updated: 0.52500 --> 0.53125
[Train] epoch: 100/2000, loss: 0.6788896918296814
[Evaluate] best accuracy performence has been updated: 0.53125 --> 0.55000
[Evaluate] best accuracy performence has been updated: 0.55000 --> 0.55625
[Evaluate] best accuracy performence has been updated: 0.55625 --> 0.56250
[Evaluate] best accuracy performence has been updated: 0.56250 --> 0.56875
[Evaluate] best accuracy performence has been updated: 0.56875 --> 0.58125
[Evaluate] best accuracy performence has been updated: 0.58125 --> 0.59375
[Evaluate] best accuracy performence has been updated: 0.59375 --> 0.60625
[Evaluate] best accuracy performence has been updated: 0.60625 --> 0.61250
[Evaluate] best accuracy performence has been updated: 0.61250 --> 0.61875
[Evaluate] best accuracy performence has been updated: 0.61875 --> 0.63750
[Evaluate] best accuracy performence has been updated: 0.63750 --> 0.64375
[Evaluate] best accuracy performence has been updated: 0.64375 --> 0.65000
[Evaluate] best accuracy performence has been updated: 0.65000 --> 0.65625
[Evaluate] best accuracy performence has been updated: 0.65625 --> 0.66875
[Evaluate] best accuracy performence has been updated: 0.66875 --> 0.67500
[Evaluate] best accuracy performence has been updated: 0.67500 --> 0.68125
[Evaluate] best accuracy performence has been updated: 0.68125 --> 0.68750
[Evaluate] best accuracy performence has been updated: 0.68750 --> 0.69375
[Evaluate] best accuracy performence has been updated: 0.69375 --> 0.70000
[Evaluate] best accuracy performence has been updated: 0.70000 --> 0.70625
[Evaluate] best accuracy performence has been updated: 0.70625 --> 0.71250
[Evaluate] best accuracy performence has been updated: 0.71250 --> 0.72500
[Evaluate] best accuracy performence has been updated: 0.72500 --> 0.73125
[Evaluate] best accuracy performence has been updated: 0.73125 --> 0.73750
[Evaluate] best accuracy performence has been updated: 0.73750 --> 0.75000
[Evaluate] best accuracy performence has been updated: 0.75000 --> 0.75625
[Evaluate] best accuracy performence has been updated: 0.75625 --> 0.76875
[Evaluate] best accuracy performence has been updated: 0.76875 --> 0.78125
[Train] epoch: 150/2000, loss: 0.6434668898582458
[Train] epoch: 200/2000, loss: 0.5647386312484741
[Evaluate] best accuracy performence has been updated: 0.78125 --> 0.78750
[Train] epoch: 250/2000, loss: 0.4727853834629059
[Evaluate] best accuracy performence has been updated: 0.78750 --> 0.79375
[Evaluate] best accuracy performence has been updated: 0.79375 --> 0.80000
[Evaluate] best accuracy performence has been updated: 0.80000 --> 0.80625
[Train] epoch: 300/2000, loss: 0.40855950117111206
[Evaluate] best accuracy performence has been updated: 0.80625 --> 0.81250
[Evaluate] best accuracy performence has been updated: 0.81250 --> 0.81875
[Evaluate] best accuracy performence has been updated: 0.81875 --> 0.82500
[Evaluate] best accuracy performence has been updated: 0.82500 --> 0.83125
[Evaluate] best accuracy performence has been updated: 0.83125 --> 0.83750
[Evaluate] best accuracy performence has been updated: 0.83750 --> 0.85000
[Evaluate] best accuracy performence has been updated: 0.85000 --> 0.85625
[Train] epoch: 350/2000, loss: 0.36973756551742554
[Evaluate] best accuracy performence has been updated: 0.85625 --> 0.86250
[Evaluate] best accuracy performence has been updated: 0.86250 --> 0.86875
[Train] epoch: 400/2000, loss: 0.3448896110057831
[Train] epoch: 450/2000, loss: 0.3279474079608917
[Evaluate] best accuracy performence has been updated: 0.86875 --> 0.87500
[Evaluate] best accuracy performence has been updated: 0.87500 --> 0.88125
[Train] epoch: 500/2000, loss: 0.3161616027355194
[Evaluate] best accuracy performence has been updated: 0.88125 --> 0.88750
[Train] epoch: 550/2000, loss: 0.30801355838775635
[Train] epoch: 600/2000, loss: 0.3024461269378662
[Evaluate] best accuracy performence has been updated: 0.88750 --> 0.89375
[Train] epoch: 650/2000, loss: 0.2986697256565094
[Evaluate] best accuracy performence has been updated: 0.89375 --> 0.90000
[Train] epoch: 700/2000, loss: 0.2961108088493347
[Train] epoch: 750/2000, loss: 0.29436975717544556
[Evaluate] best accuracy performence has been updated: 0.90000 --> 0.90625
[Train] epoch: 800/2000, loss: 0.2931761145591736
[Train] epoch: 850/2000, loss: 0.2923488914966583
[Train] epoch: 900/2000, loss: 0.291767954826355
[Train] epoch: 950/2000, loss: 0.29135289788246155
[Train] epoch: 1000/2000, loss: 0.29105034470558167
[Train] epoch: 1050/2000, loss: 0.290824294090271
[Train] epoch: 1100/2000, loss: 0.29065054655075073
[Train] epoch: 1150/2000, loss: 0.2905128598213196
[Train] epoch: 1200/2000, loss: 0.29040008783340454
[Train] epoch: 1250/2000, loss: 0.29030489921569824
[Train] epoch: 1300/2000, loss: 0.2902221083641052
[Train] epoch: 1350/2000, loss: 0.29014816880226135
[Train] epoch: 1400/2000, loss: 0.290080726146698
[Train] epoch: 1450/2000, loss: 0.290018230676651
[Train] epoch: 1500/2000, loss: 0.2899594306945801
[Train] epoch: 1550/2000, loss: 0.28990358114242554
[Train] epoch: 1600/2000, loss: 0.2898501753807068
[Train] epoch: 1650/2000, loss: 0.2897987961769104
[Train] epoch: 1700/2000, loss: 0.2897491455078125
[Train] epoch: 1750/2000, loss: 0.28970101475715637
[Train] epoch: 1800/2000, loss: 0.2896542549133301
[Train] epoch: 1850/2000, loss: 0.28960874676704407
[Train] epoch: 1900/2000, loss: 0.28956443071365356
[Train] epoch: 1950/2000, loss: 0.28952115774154663
[Test] score/loss: 0.9050/0.2641

增加一个3个神经元的隐藏层，再次实现二分类，并与1做对比。

改为3个神经元的隐藏层，我们需要增加一个隐藏层的参数初始化：

input_size = 2
hidden_size = 5
# 增加
hidden_size2 = 3
output_size = 1
model = Model_MLP_L2_V4(input_size=input_size, hidden_size=hidden_size,hidden_size2=hidden_size2, output_size=output_size)

模型上要增加函数


class Model_MLP_L2_V4(torch.nn.Module):
    def __init__(self, input_size, hidden_size, hidden_size2, output_size):
        super(Model_MLP_L2_V4, self).__init__()
        self.fc1 = nn.Linear(input_size, hidden_size)
        w1=torch.normal(0,0.1,size=(hidden_size,input_size),requires_grad=True)
        self.fc1.weight = nn.Parameter(w1)

        self.fc2 = nn.Linear(hidden_size, hidden_size2)
        w2 = torch.normal(0, 0.1, size=(hidden_size2, hidden_size), requires_grad=True)
        self.fc2.weight = nn.Parameter(w2)

        self.fc3 = nn.Linear(hidden_size2, output_size)
        w3 = torch.normal(0, 0.1, size=(output_size, hidden_size2), requires_grad=True)
        self.fc3.weight = nn.Parameter(w3)

        # 使用"torch.nn.functional.sigmoid"定义 Logistic 激活函数
        self.act_fn = torch.sigmoid

    # 前向计算
    def forward(self, inputs):
        z1 = self.fc1(inputs.to(torch.float32))
        a1 = self.act_fn(z1)
        z2 = self.fc2(a1)
        a2 = self.act_fn(z2)
        z3 = self.fc3(a2)
        a3 = self.act_fn(z3)
        return a3

运行结果：

[Evaluate] best accuracy performence has been updated: 0.00000 --> 0.51250
[Train] epoch: 0/2000, loss: 0.6943337321281433
[Train] epoch: 50/2000, loss: 0.6928403973579407
[Train] epoch: 100/2000, loss: 0.6928308606147766
[Train] epoch: 150/2000, loss: 0.6928211450576782
[Train] epoch: 200/2000, loss: 0.6928109526634216
[Train] epoch: 250/2000, loss: 0.6928002238273621
[Train] epoch: 300/2000, loss: 0.6927885413169861
[Train] epoch: 350/2000, loss: 0.692775547504425
[Train] epoch: 400/2000, loss: 0.6927613019943237
[Train] epoch: 450/2000, loss: 0.6927453875541687
[Train] epoch: 500/2000, loss: 0.6927274465560913
[Train] epoch: 550/2000, loss: 0.6927071213722229
[Train] epoch: 600/2000, loss: 0.6926836967468262
[Train] epoch: 650/2000, loss: 0.6926565170288086
[Train] epoch: 700/2000, loss: 0.6926250457763672
[Train] epoch: 750/2000, loss: 0.692588210105896
[Train] epoch: 800/2000, loss: 0.6925449371337891
[Train] epoch: 850/2000, loss: 0.6924934983253479
[Train] epoch: 900/2000, loss: 0.6924319267272949
[Train] epoch: 950/2000, loss: 0.6923579573631287
[Train] epoch: 1000/2000, loss: 0.6922681927680969
[Train] epoch: 1050/2000, loss: 0.6921581029891968
[Train] epoch: 1100/2000, loss: 0.6920220255851746
[Train] epoch: 1150/2000, loss: 0.691851794719696
[Train] epoch: 1200/2000, loss: 0.6916364431381226
[Train] epoch: 1250/2000, loss: 0.6913599967956543
[Train] epoch: 1300/2000, loss: 0.6909999847412109
[Train] epoch: 1350/2000, loss: 0.6905229687690735
[Train] epoch: 1400/2000, loss: 0.6898793578147888
[Train] epoch: 1450/2000, loss: 0.688992440700531
[Train] epoch: 1500/2000, loss: 0.6877409219741821
[Train] epoch: 1550/2000, loss: 0.6859274506568909
[Evaluate] best accuracy performence has been updated: 0.51250 --> 0.51875
[Evaluate] best accuracy performence has been updated: 0.51875 --> 0.52500
[Evaluate] best accuracy performence has been updated: 0.52500 --> 0.53750
[Evaluate] best accuracy performence has been updated: 0.53750 --> 0.54375
[Evaluate] best accuracy performence has been updated: 0.54375 --> 0.55625
[Evaluate] best accuracy performence has been updated: 0.55625 --> 0.56875
[Evaluate] best accuracy performence has been updated: 0.56875 --> 0.57500
[Evaluate] best accuracy performence has been updated: 0.57500 --> 0.59375
[Evaluate] best accuracy performence has been updated: 0.59375 --> 0.60000
[Evaluate] best accuracy performence has been updated: 0.60000 --> 0.60625
[Evaluate] best accuracy performence has been updated: 0.60625 --> 0.61875
[Evaluate] best accuracy performence has been updated: 0.61875 --> 0.62500
[Evaluate] best accuracy performence has been updated: 0.62500 --> 0.63125
[Evaluate] best accuracy performence has been updated: 0.63125 --> 0.63750
[Train] epoch: 1600/2000, loss: 0.6832197904586792
[Evaluate] best accuracy performence has been updated: 0.63750 --> 0.65000
[Evaluate] best accuracy performence has been updated: 0.65000 --> 0.65625
[Evaluate] best accuracy performence has been updated: 0.65625 --> 0.66875
[Evaluate] best accuracy performence has been updated: 0.66875 --> 0.68125
[Evaluate] best accuracy performence has been updated: 0.68125 --> 0.69375
[Evaluate] best accuracy performence has been updated: 0.69375 --> 0.70000
[Evaluate] best accuracy performence has been updated: 0.70000 --> 0.70625
[Evaluate] best accuracy performence has been updated: 0.70625 --> 0.71250
[Evaluate] best accuracy performence has been updated: 0.71250 --> 0.71875
[Evaluate] best accuracy performence has been updated: 0.71875 --> 0.72500
[Evaluate] best accuracy performence has been updated: 0.72500 --> 0.73125
[Evaluate] best accuracy performence has been updated: 0.73125 --> 0.73750
[Evaluate] best accuracy performence has been updated: 0.73750 --> 0.74375
[Evaluate] best accuracy performence has been updated: 0.74375 --> 0.75000
[Evaluate] best accuracy performence has been updated: 0.75000 --> 0.75625
[Evaluate] best accuracy performence has been updated: 0.75625 --> 0.76250
[Evaluate] best accuracy performence has been updated: 0.76250 --> 0.76875
[Evaluate] best accuracy performence has been updated: 0.76875 --> 0.77500
[Evaluate] best accuracy performence has been updated: 0.77500 --> 0.78125
[Evaluate] best accuracy performence has been updated: 0.78125 --> 0.79375
[Evaluate] best accuracy performence has been updated: 0.79375 --> 0.80000
[Evaluate] best accuracy performence has been updated: 0.80000 --> 0.80625
[Evaluate] best accuracy performence has been updated: 0.80625 --> 0.81250
[Train] epoch: 1650/2000, loss: 0.6790398955345154
[Evaluate] best accuracy performence has been updated: 0.81250 --> 0.81875
[Evaluate] best accuracy performence has been updated: 0.81875 --> 0.82500
[Evaluate] best accuracy performence has been updated: 0.82500 --> 0.83125
[Evaluate] best accuracy performence has been updated: 0.83125 --> 0.83750
[Evaluate] best accuracy performence has been updated: 0.83750 --> 0.84375
[Train] epoch: 1700/2000, loss: 0.6723501086235046
[Train] epoch: 1750/2000, loss: 0.661268413066864
[Train] epoch: 1800/2000, loss: 0.6425549387931824
[Train] epoch: 1850/2000, loss: 0.6118574142456055
[Train] epoch: 1900/2000, loss: 0.5674184560775757
[Train] epoch: 1950/2000, loss: 0.515984296798706
[Test] score/loss: 0.8100/0.6730

这里我们可以发现，训练模型在测试集上的准确率得到了进一步的提升。

【思考题】自定义梯度计算和自动梯度计算：从计算性能、计算结果等多方面比较，谈谈自己的看法。

使用前面实验Model_MLP_L2类

激活函数：

def backward(self, grads):
	outputs_grad_inputs = torch.multiply(self.outputs, (1.0 - self.outputs))
	return torch.multiply(grads, outputs_grad_inputs)

损失函数：

def backward(self):
	loss_grad_predicts = -1.0 * (self.labels / self.predicts - (1 - self.labels) / (1 - self.predicts)) / self.num
	self.model.backward(loss_grad_predicts)

自动梯度求导：自定义梯度求导：

自定义梯度计算是一步到位，计算速度很快对比了一下，发现二者更新的结果还是有很大出入的，不仅仅体现在测试集准确率上，这权重梯度也很不一样。在时间充裕爱好数学的方式下，尽量采用自定义求导。

需要导入的包

文章为作者独立观点，不代表观点

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