第9.3讲、Tiny Transformer: 极简版Transformer

简介

极简版的 Transformer 编码器-解码器（Seq2Seq）结构，适合用于学习、实验和小型序列到序列（如翻译、摘要）任务。
该实现包含了位置编码、多层编码器、多层解码器、训练与推理流程，代码简洁易懂，便于理解 Transformer 的基本原理。

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主要结构

• PositionalEncoding：正弦/余弦位置编码，为输入embedding添加位置信息。
• TransformerEncoderLayerWithTrace：单层编码器，含自注意力和前馈网络。
• TinyTransformer：多层堆叠的编码器。
• TransformerDecoderLayer：单层解码器，含自注意力、交叉注意力和前馈网络。
• TransformerDecoder：多层堆叠的解码器。
• TinyTransformerSeq2Seq：编码器-解码器整体结构。
• Seq2SeqDataset：简单的序列到序列数据集。
• train：训练循环。
• greedy_decode：贪婪解码推理。
• generate_subsequent_mask：生成自回归mask。

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依赖环境

• Python 3.7+
• torch >= 1.10

安装 PyTorch（以 CPU 版本为例）：

pip install torch

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用法示例

1. 构建模型

from demo import TinyTransformerSeq2Seq

src_vocab_size = 1000
trg_vocab_size = 1000
model = TinyTransformerSeq2Seq(src_vocab_size, trg_vocab_size)

2. 构造数据集和 DataLoader

from demo import Seq2SeqDataset
from torch.utils.data import DataLoader

src_data = torch.randint(0, src_vocab_size, (100, 10))  # 100个样本，每个10个token
trg_data = torch.randint(0, trg_vocab_size, (100, 12))  # 100个样本，每个12个token

dataset = Seq2SeqDataset(src_data, trg_data)
dataloader = DataLoader(dataset, batch_size=16, shuffle=True)

3. 训练模型

import torch.optim as optim
import torch.nn as nn
from demo import train

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
model = model.to(device)
optimizer = optim.Adam(model.parameters(), lr=1e-3)
criterion = nn.CrossEntropyLoss(ignore_index=0)

epoch_loss = train(model, dataloader, optimizer, criterion, tgt_pad_idx=0)
print(f"Train loss: {epoch_loss}")

4. 推理（贪婪解码）

from demo import greedy_decode

src = torch.randint(0, src_vocab_size, (1, 10)).to(device)
sos_idx = 1  # 假设1为<sos>
eos_idx = 2  # 假设2为<eos>
max_len = 12
output = greedy_decode(model, src, sos_idx, eos_idx, max_len)
print("Output token ids:", output)

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注意事项

• 该实现为教学/实验用途，未包含完整的mask、权重初始化、分布式训练等工业级细节。
• 需要自行准备合适的训练数据和词表。
• 若需工业级NLP任务，建议使用 HuggingFace Transformers。

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tiny Transformer案例代码1：

import torch
import torch.nn as nn
import torch.optim as optim
import math
from torch.utils.data import DataLoader, Dataset

# =========================
# 位置编码模块
# =========================
class PositionalEncoding(nn.Module):
    """
    为输入的embedding添加位置信息，帮助模型捕捉序列顺序。
    """
    def __init__(self, d_model, max_len=512):
        super().__init__()
        pe = torch.zeros(max_len, d_model)
        position = torch.arange(0, max_len).unsqueeze(1).float()
        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)
        pe = pe.unsqueeze(0)  # (1, max_len, d_model)
        self.register_buffer('pe', pe)

    def forward(self, x):
        # x: (batch, seq_len, d_model)
        return x + self.pe[:, :x.size(1)]

# =========================
# 编码器层
# =========================
class TransformerEncoderLayerWithTrace(nn.Module):
    """
    单层Transformer编码器，带自注意力和前馈网络。
    """
    def __init__(self, d_model, nhead, dim_feedforward):
        super().__init__()
        self.self_attn = nn.MultiheadAttention(d_model, nhead, batch_first=True)
        self.linear1 = nn.Linear(d_model, dim_feedforward)
        self.linear2 = nn.Linear(dim_feedforward, d_model)
        self.norm1 = nn.LayerNorm(d_model)
        self.norm2 = nn.LayerNorm(d_model)
        self.dropout = nn.Dropout(0.1)

    def forward(self, src, src_mask=None):
        # src: (batch, seq_len, d_model)
        src2, attn_weights = self.self_attn(src, src, src, attn_mask=src_mask)
        src = self.norm1(src + self.dropout(src2))
        src2 = self.linear2(self.dropout(torch.relu(self.linear1(src))))
        src = self.norm2(src + self.dropout(src2))
        return src, attn_weights

# =========================
# 编码器
# =========================
class TinyTransformer(nn.Module):
    """
    多层堆叠的Transformer编码器。
    """
    def __init__(self, src_vocab_size, tgt_vocab_size, d_model, nhead, dim_feedforward, max_len, num_layers):
        super().__init__()
        self.embedding = nn.Embedding(src_vocab_size, d_model)
        self.pos_encoder = PositionalEncoding(d_model, max_len)
        self.layers = nn.ModuleList([
            TransformerEncoderLayerWithTrace(d_model, nhead, dim_feedforward)
            for _ in range(num_layers)
        ])

    def forward(self, src, trace=False):
        # src: (batch, seq_len)
        src = self.embedding(src)
        src = self.pos_encoder(src)
        attn_weights_all = []
        for layer in self.layers:
            src, attn_weights = layer(src)
            if trace:
                attn_weights_all.append(attn_weights)
        return src, attn_weights_all

# =========================
# 解码器层
# =========================
class TransformerDecoderLayer(nn.Module):
    """
    单层Transformer解码器，含自注意力、交叉注意力和前馈网络。
    """
    def __init__(self, d_model, nhead, dim_feedforward):
        super().__init__()
        self.self_attn = nn.MultiheadAttention(d_model, nhead, batch_first=True)
        self.cross_attn = nn.MultiheadAttention(d_model, nhead, batch_first=True)
        self.linear1 = nn.Linear(d_model, dim_feedforward)
        self.linear2 = nn.Linear(dim_feedforward, d_model)
        self.norm1 = nn.LayerNorm(d_model)
        self.norm2 = nn.LayerNorm(d_model)
        self.norm3 = nn.LayerNorm(d_model)
        self.dropout = nn.Dropout(0.1)

    def forward(self, tgt, memory, tgt_mask=None, memory_mask=None):
        # tgt: (batch, tgt_seq_len, d_model)
        # memory: (batch, src_seq_len, d_model)
        tgt2, _ = self.self_attn(tgt, tgt, tgt, attn_mask=tgt_mask)
        tgt = self.norm1(tgt + self.dropout(tgt2))

        tgt2, _ = self.cross_attn(tgt, memory, memory, attn_mask=memory_mask)
        tgt = self.norm2(tgt + self.dropout(tgt2))

        tgt2 = self.linear2(self.dropout(torch.relu(self.linear1(tgt))))
        tgt = self.norm3(tgt + self.dropout(tgt2))
        return tgt

# =========================
# 解码器
# =========================
class TransformerDecoder(nn.Module):
    """
    多层堆叠的Transformer解码器。
    """
    def __init__(self, vocab_size, d_model, nhead, dim_feedforward, num_layers, max_len=512):
        super().__init__()
        self.embedding = nn.Embedding(vocab_size, d_model)
        self.pos_encoder = PositionalEncoding(d_model, max_len)
        self.layers = nn.ModuleList([
            TransformerDecoderLayer(d_model, nhead, dim_feedforward)
            for _ in range(num_layers)
        ])
        self.out_proj = nn.Linear(d_model, vocab_size)

    def forward(self, tgt_ids, memory, tgt_mask=None):
        # tgt_ids: (batch, tgt_seq_len)
        x = self.embedding(tgt_ids)
        x = self.pos_encoder(x)
        for layer in self.layers:
            x = layer(x, memory, tgt_mask)
        return self.out_proj(x)

# =========================
# Seq2Seq整体模型
# =========================
class TinyTransformerSeq2Seq(nn.Module):
    """
    编码器-解码器结构的Transformer模型。
    """
    def __init__(self, src_vocab_size, tgt_vocab_size, d_model=64, heads=4, d_ff=128, num_layers=2, max_len=64):
        super().__init__()
        self.encoder = TinyTransformer(src_vocab_size, tgt_vocab_size, d_model, heads, d_ff, max_len, num_layers)
        self.decoder = TransformerDecoder(
            vocab_size=tgt_vocab_size,
            d_model=d_model,
            nhead=heads,
            dim_feedforward=d_ff,
            num_layers=num_layers,
            max_len=max_len
        )

    def forward(self, src_ids, tgt_input_ids, tgt_mask=None):
        # src_ids: (batch, src_seq_len)
        # tgt_input_ids: (batch, tgt_seq_len)
        memory, _ = self.encoder(src_ids, trace=False)
        logits = self.decoder(tgt_input_ids, memory, tgt_mask)
        return logits

# =========================
# 工具函数: 生成自回归mask
# =========================
def generate_subsequent_mask(size):
    """
    生成自回归mask，防止解码器看到未来的信息。
    """
    return torch.triu(torch.full((size, size), float('-inf')), diagonal=1)

# =========================
# Toy数据集
# =========================
class Seq2SeqDataset(Dataset):
    """
    简单的序列到序列数据集。
    """
    def __init__(self, src_data, tgt_data):
        self.src = src_data
        self.tgt = tgt_data

    def __len__(self):
        return len(self.src)

    def __getitem__(self, idx):
        return self.src[idx], self.tgt[idx]

# =========================
# 训练循环
# =========================
def train(model, dataloader, optimizer, criterion, tgt_pad_idx):
    """
    训练模型一个epoch。
    """
    model.train()
    total_loss = 0
    for src, tgt in dataloader:
        src, tgt = src.to(device), tgt.to(device)
        tgt_input = tgt[:, :-1]
        tgt_output = tgt[:, 1:]

        tgt_mask = generate_subsequent_mask(tgt_input.size(1)).to(device)
        logits = model(src, tgt_input, tgt_mask)

        loss = criterion(logits.view(-1, logits.size(-1)), tgt_output.reshape(-1))
        optimizer.zero_grad()
        loss.backward()
        optimizer.step()
        total_loss += loss.item()
    return total_loss / len(dataloader)

# =========================
# 推理（贪婪解码）
# =========================
def greedy_decode(model, src, sos_idx, eos_idx, max_len):
    """
    贪婪解码：每步选择概率最大的token，直到eos或最大长度。
    """
    model.eval()
    src = src.to(device)
    memory, _ = model.encoder(src, trace=False)
    tgt = torch.ones((src.size(0), 1), dtype=torch.long).fill_(sos_idx).to(device)

    for _ in range(max_len - 1):
        tgt_mask = generate_subsequent_mask(tgt.size(1)).to(device)
        out = model.decoder(tgt, memory, tgt_mask)
        next_token = out[:, -1, :].argmax(dim=-1).unsqueeze(1)
        tgt = torch.cat([tgt, next_token], dim=1)
        if (next_token == eos_idx).all():
            break
    return tgt

# =========================
# 设备选择
# =========================
device = torch.device("cuda" if torch.cuda.is_available() else "cpu")