一、nn.Modules

我们可以定义一个模型，这个模型继承自nn.Module类。如果需要定义一个比Sequential模型更加复杂的模型，就需要定义nn.Module模型。
定义了__init__和 forward 两个方法，就实现了自定义的网络模型。
_init_(),定义模型架构，实现每个层的定义。
forward(),实现前向传播，返回y_pred

import torch


class TwoLayerNet(torch.nn.Module):
    def __init__(self, D_in, H, D_out):
        """
        In the constructor we instantiate two nn.Linear modules and assign them as
        member variables.
        """
        super(TwoLayerNet, self).__init__()
        self.linear1 = torch.nn.Linear(D_in, H)
        self.linear2 = torch.nn.Linear(H, D_out)

    def forward(self, x):
        """
        In the forward function we accept a Tensor of input data and we must return
        a Tensor of output data. We can use Modules defined in the constructor as
        well as arbitrary operators on Tensors.
        """
        h_relu = self.linear1(x).clamp(min=0)
        y_pred = self.linear2(h_relu)
        return y_pred



N, D_in, H, D_out = 64, 1000, 100, 10
x = torch.randn(N, D_in)
y = torch.randn(N, D_out)

model = TwoLayerNet(D_in, H, D_out)

criterion = torch.nn.MSELoss(reduction='sum')
optimizer = torch.optim.SGD(model.parameters(), lr=1e-4)
for t in range(500):
   
    y_pred = model(x)
    loss = criterion(y_pred, y)
    print(t, loss.item())
    optimizer.zero_grad()
    loss.backward()
    optimizer.step()

二、一个实例:FizzBuzz

FizzBuzz是一个简单的小游戏。游戏规则如下：从1开始往上数数，当遇到3的倍数的时候，说fizz，当遇到5的倍数，说buzz，当遇到15的倍数，就说fizzbuzz，其他情况下则正常数数。

# One-hot encode the desired outputs: [number, "fizz", "buzz", "fizzbuzz"]
def fizz_buzz_encode(i):
    if   i % 15 == 0: return 3
    elif i % 5  == 0: return 2
    elif i % 3  == 0: return 1
    else:             return 0
    
def fizz_buzz_decode(i, prediction):
    return [str(i), "fizz", "buzz", "fizzbuzz"][prediction]

首先定义模型的输入与输出(训练数据)

import numpy as np
import torch

NUM_DIGITS = 10

# Represent each input by an array of its binary digits.
def binary_encode(i, num_digits):
    return np.array([i >> d & 1 for d in range(num_digits)])[::-1] # 右移一位再和1做与运算。
# 右移动运算符：把">>"左边的运算数的各二进位全部右移若干位，>> 右边的数字指定了移动的位数 
trX = torch.Tensor([binary_encode(i, NUM_DIGITS) for i in range(101, 2 ** NUM_DIGITS)])
trY = torch.LongTensor([fizz_buzz_encode(i) for i in range(101, 2 ** NUM_DIGITS)]) #因为表示类别，用LongTensor

然后用PyTorch定义模型,损失函数，优化器。

# Define the model
NUM_HIDDEN = 100
model = torch.nn.Sequential(
    torch.nn.Linear(NUM_DIGITS, NUM_HIDDEN),
    torch.nn.ReLU(),
    torch.nn.Linear(NUM_HIDDEN, 4)
)
loss_fn = torch.nn.CrossEntropyLoss()
optimizer = torch.optim.SGD(model.parameters(), lr = 0.05)

以下是模型的训练代码

# Start training it
BATCH_SIZE = 128
for epoch in range(10000):
    for start in range(0, len(trX), BATCH_SIZE):
        end = start + BATCH_SIZE
        batchX = trX[start:end]
        batchY = trY[start:end]

        y_pred = model(batchX)
        loss = loss_fn(y_pred, batchY)

        optimizer.zero_grad()
        loss.backward()
        optimizer.step()

    # Find loss on training data
    loss = loss_fn(model(trX), trY).item()
    print('Epoch:', epoch, 'Loss:', loss)