用 Pytorch 训练一个 Transformer模型

昨天说了一下Transformer架构，今天我们来看看怎么 Pytorch 训练一个Transormer模型，真实训练一个模型是个庞大工程，准备数据、准备硬件等等，我只是做一个简单的实现。因为只是做实验，本地用 CPU 也可以运行。
本文包含以下几部分：

1. 准备环境。
2. 然后就是跟据架构来定义每一层，包括Embedding、Position Encoding、多头注意力、 网络层。
3. 准备Encoder。
4. 准备Decoder。
5. 运行 Transformer，包括训练和评估。

安装Pytorch 环境

!pip3 install torch torchvision torchaudio
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引入所需工具类库

引入需要的类库，pytorch 是强大的训练框架，深度学习中需要的一些函数和基本功能都已经实现。

import torch
import torch.nn as nn
import torch.optim as optim
import torch.utils.data as data
import math
import copy
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Position Embedding

生成位置信息。

class PositionalEncoding(nn.Module):
    def __init__(self, d_model, max_seq_length):
        super(PositionalEncoding, self).__init__()
        
        pe = torch.zeros(max_seq_length, d_model)
        position = torch.arange(0, max_seq_length, dtype=torch.float).unsqueeze(1)
        div_term = torch.exp(torch.arange(0, d_model, 2).float() * -(math.log(10000.0) / d_model))
        
        pe[:, 0::2] = torch.sin(position * div_term)
        pe[:, 1::2] = torch.cos(position * div_term)
        
        self.register_buffer('pe', pe.unsqueeze(0))
        
    def forward(self, x):
        return x + self.pe[:, :x.size(1)]
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多头注意力

1. 初始化model 维度，头数，每个头的维度。
2. 计算是在 forward 这个方法，主要看这个方法。

class MultiHeadAttention(nn.Module):
    def __init__(self, d_model, num_heads):
        super(MultiHeadAttention, self).__init__()
        # Ensure that the model dimension (d_model) is divisible by the number of heads
        assert d_model % num_heads == 0, "d_model must be divisible by num_heads"
        
        # Initialize dimensions
        self.d_model = d_model # Model's dimension
        self.num_heads = num_heads # Number of attention heads
        self.d_k = d_model // num_heads # Dimension of each head's key, query, and value
        
        # Linear layers for transforming inputs
        self.W_q = nn.Linear(d_model, d_model) # Query transformation
        self.W_k = nn.Linear(d_model, d_model) # Key transformation
        self.W_v = nn.Linear(d_model, d_model) # Value transformation
        self.W_o = nn.Linear(d_model, d_model) # Output transformation
        
    def scaled_dot_product_attention(self, Q, K, V, mask=None):
        # Calculate attention scores
        attn_scores = torch.matmul(Q, K.transpose(-2, -1)) / math.sqrt(self.d_k)
        
        # Apply mask if provided (useful for preventing attention to certain parts like padding)
        if mask is not None:
            attn_scores = attn_scores.masked_fill(mask == 0, -1e9)
        
        # Softmax is applied to obtain attention probabilities
        attn_probs = torch.softmax(attn_scores, dim=-1)
        
        # Multiply by values to obtain the final output
        output = torch.matmul(attn_probs, V)
        return output
        
    def split_heads(self, x):
        # 转换，每一个 head 独立处理
        batch_size, seq_length, d_model = x.size()
        return x.view(batch_size, seq_length, self.num_heads, self.d_k).transpose(1, 2)
        
    def combine_heads(self, x):
        # Combine the multiple heads back to original shape
        batch_size, _, seq_length, d_k = x.size()
        return x.transpose(1, 2).contiguous().view(batch_size, seq_length, self.d_model)
        
    def forward(self, Q, K, V, mask=None):
        # 线性转换并切分
        Q = self.split_heads(self.W_q(Q))
        K = self.split_heads(self.W_k(K))
        V = self.split_heads(self.W_v(V))
        
        # 运行计算公式
        attn_output = self.scaled_dot_product_attention(Q, K, V, mask)
        
        # 合并并返回
        output = self.W_o(self.combine_heads(attn_output))
        return output
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网络定义

class PositionWiseFeedForward(nn.Module):
    def __init__(self, d_model, d_ff):
        super(PositionWiseFeedForward, self).__init__()
        self.fc1 = nn.Linear(d_model, d_ff)
        self.fc2 = nn.Linear(d_ff, d_model)
        self.relu = nn.ReLU()

    def forward(self, x):
        return self.fc2(self.relu(self.fc1(x)))
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Encoder

跟据这张图看下面的实现比较直观，初始化了MultiHeadAttention、PositionWiseFeedForward、两个LayerNorm。 forward 方法中 x 是 Encoder 的输入。

class EncoderLayer(nn.Module):
    def __init__(self, d_model, num_heads, d_ff, dropout):
        super(EncoderLayer, self).__init__()
        self.self_attn = MultiHeadAttention(d_model, num_heads)
        self.feed_forward = PositionWiseFeedForward(d_model, d_ff)
        self.norm1 = nn.LayerNorm(d_model)
        self.norm2 = nn.LayerNorm(d_model)
        self.dropout = nn.Dropout(dropout)
        
    def forward(self, x, mask):
        attn_output = self.self_attn(x, x, x, mask)
        x = self.norm1(x + self.dropout(attn_output))
        ff_output = self.feed_forward(x)
        x = self.norm2(x + self.dropout(ff_output))
        return x
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Decoder

看代码的方式和 Encoder 类似，比较好理解，2 个MultiHeadAttention、3个 Norm，forward 中cross_attn 把 enc_output作为传入的参数。

class DecoderLayer(nn.Module):
    def __init__(self, d_model, num_heads, d_ff, dropout):
        super(DecoderLayer, self).__init__()
        self.self_attn = MultiHeadAttention(d_model, num_heads)
        self.cross_attn = MultiHeadAttention(d_model, num_heads)
        self.feed_forward = PositionWiseFeedForward(d_model, d_ff)
        self.norm1 = nn.LayerNorm(d_model)
        self.norm2 = nn.LayerNorm(d_model)
        self.norm3 = nn.LayerNorm(d_model)
        self.dropout = nn.Dropout(dropout)
        
    def forward(self, x, enc_output, src_mask, tgt_mask):
        attn_output = self.self_attn(x, x, x, tgt_mask)
        x = self.norm1(x + self.dropout(attn_output))
        attn_output = self.cross_attn(x, enc_output, enc_output, src_mask)
        x = self.norm2(x + self.dropout(attn_output))
        ff_output = self.feed_forward(x)
        x = self.norm3(x + self.dropout(ff_output))
        return x
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Transformer

Transformer主类，包括初始化 embedding、position embedding、encoder 和 decoder。forward 方法进行计算。

class Transformer(nn.Module):
    def __init__(self, src_vocab_size, tgt_vocab_size, d_model, num_heads, num_layers, d_ff, max_seq_length, dropout):
        super(Transformer, self).__init__()
        self.encoder_embedding = nn.Embedding(src_vocab_size, d_model)
        self.decoder_embedding = nn.Embedding(tgt_vocab_size, d_model)
        self.positional_encoding = PositionalEncoding(d_model, max_seq_length)

        self.encoder_layers = nn.ModuleList([EncoderLayer(d_model, num_heads, d_ff, dropout) for _ in range(num_layers)])
        self.decoder_layers = nn.ModuleList([DecoderLayer(d_model, num_heads, d_ff, dropout) for _ in range(num_layers)])

        self.fc = nn.Linear(d_model, tgt_vocab_size)
        self.dropout = nn.Dropout(dropout)

    def generate_mask(self, src, tgt):
        src_mask = (src != 0).unsqueeze(1).unsqueeze(2)
        tgt_mask = (tgt != 0).unsqueeze(1).unsqueeze(3)
        seq_length = tgt.size(1)
        nopeak_mask = (1 - torch.triu(torch.ones(1, seq_length, seq_length), diagonal=1)).bool()
        tgt_mask = tgt_mask & nopeak_mask
        return src_mask, tgt_mask

    def forward(self, src, tgt):
        src_mask, tgt_mask = self.generate_mask(src, tgt)
        src_embedded = self.dropout(self.positional_encoding(self.encoder_embedding(src)))
        tgt_embedded = self.dropout(self.positional_encoding(self.decoder_embedding(tgt)))

        enc_output = src_embedded
        for enc_layer in self.encoder_layers:
            enc_output = enc_layer(enc_output, src_mask)

        dec_output = tgt_embedded
        for dec_layer in self.decoder_layers:
            dec_output = dec_layer(dec_output, enc_output, src_mask, tgt_mask)

        output = self.fc(dec_output)
        return output
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训练

首先准备数据，这里的数据是随机生成，只是做演示。

src_vocab_size = 5000
tgt_vocab_size = 5000
d_model = 512
num_heads = 8
num_layers = 6
d_ff = 2048
max_seq_length = 100
dropout = 0.1

transformer = Transformer(src_vocab_size, tgt_vocab_size, d_model, num_heads, num_layers, d_ff, max_seq_length, dropout)

# Generate random sample data
src_data = torch.randint(1, src_vocab_size, (64, max_seq_length))  # (batch_size, seq_length)
tgt_data = torch.randint(1, tgt_vocab_size, (64, max_seq_length))  # (batch_size, seq_length)
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开始训练

criterion = nn.CrossEntropyLoss(ignore_index=0)
optimizer = optim.Adam(transformer.parameters(), lr=0.0001, betas=(0.9, 0.98), eps=1e-9)

transformer.train()

for epoch in range(100):
    optimizer.zero_grad()
    output = transformer(src_data, tgt_data[:, :-1])
    loss = criterion(output.contiguous().view(-1, tgt_vocab_size), tgt_data[:, 1:].contiguous().view(-1))
    loss.backward()
    optimizer.step()
    print(f"Epoch: {epoch+1}, Loss: {loss.item()}")
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评估

将模型运行在验证集或者测试集上，这里数据也是随机生成的，只为体验一下完整流程。

transformer.eval()

# Generate random sample validation data
val_src_data = torch.randint(1, src_vocab_size, (64, max_seq_length))  # (batch_size, seq_length)
val_tgt_data = torch.randint(1, tgt_vocab_size, (64, max_seq_length))  # (batch_size, seq_length)

with torch.no_grad():

    val_output = transformer(val_src_data, val_tgt_data[:, :-1])
    val_loss = criterion(val_output.contiguous().view(-1, tgt_vocab_size), val_tgt_data[:, 1:].contiguous().view(-1))
    print(f"Validation Loss: {val_loss.item()}")
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如果你对 Pytorch 和神经网络比较熟悉，Transformer整体实现起来并不复杂，如果想我一样对深度学习不太熟悉，理解起来还是有些困难，这里只是大概跑了一下流程，对Transformer训练有一个概念。