权重衰退实验（李沐动手学）

import torch
from torch import nn
from d2l import torch as d2l
import matplotlib.pyplot as plt
n_train, n_test, num_inputs, batch_size = 20, 100, 200, 5
true_w, true_b = torch.ones((num_inputs, 1)) * 0.01, 0.05
train_data = d2l.synthetic_data(true_w, true_b, n_train)
train_iter = d2l.load_array(train_data, batch_size)
test_data = d2l.synthetic_data(true_w, true_b, n_test)
test_iter = d2l.load_array(test_data, batch_size, is_train=False)
def init_params():
    w=torch.normal(0,1,(num_inputs,1),requires_grad=True)
    b=torch.zeros(1,requires_grad=True)
    return [w,b]
def l2_penalty(w):
    return torch.sum(w.pow(2))/2

def train(lambd):

    num_epochs=100
    lr=0.01
    animator = d2l.Animator(xlabel='epochs', ylabel='loss', yscale='log',
                            xlim=[5, num_epochs], legend=['train', 'test'])

    w,b=init_params()
    net=lambda X:d2l.linreg(X,w,b)
    loss=d2l.squared_loss
    for epoch in range(num_epochs):
        for X,y in train_iter:
            l=loss(net(X),y)+float(lambd)*l2_penalty(w)
            l.sum().backward()
            d2l.sgd([w,b],lr,batch_size)
        if (epoch+1)%5==0:
            animator.add(epoch + 1, (d2l.evaluate_loss(net, train_iter, loss),
                                     d2l.evaluate_loss(net, test_iter, loss)))

    print("w的l2范数是:",torch.norm(w).item())

train(lambd=0)
train(lambd=3)
plt.show()

无权重衰退
有权重衰退