Codes/ch03/01_main-chapter-code/multihead-attention.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "source": [
    "# 多头注意力机制结合数据加载"
   ],
   "metadata": {
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   },
   "id": "6c73d8caae2916ec"
  },
  {
   "cell_type": "markdown",
   "source": [
    "完整的章节代码位于 [ch03.ipynb](./ch03.ipynb)。\n",
    "\n",
    "这个 notebook 包含了主要的学习点，即多头注意力的实现（以及第二章中的数据加载流程）。"
   ],
   "metadata": {
    "collapsed": false
   },
   "id": "660c1de13c8952cf"
  },
  {
   "cell_type": "markdown",
   "source": [
    "## 第二章中的数据加载器"
   ],
   "metadata": {
    "collapsed": false
   },
   "id": "62c346f743383e17"
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  {
   "cell_type": "code",
   "execution_count": 1,
   "outputs": [],
   "source": [
    "# 导入必要的库\n",
    "import tiktoken  # 假设这是一个自定义的分词库\n",
    "import torch\n",
    "import torch.nn as nn\n",
    "from torch.utils.data import Dataset, DataLoader\n",
    "\n",
    "# 定义 GPT 数据集类\n",
    "class GPTDatasetV1(Dataset):\n",
    "    def __init__(self, txt, tokenizer, max_length, stride):\n",
    "        # 初始化分词器\n",
    "        self.tokenizer = tokenizer\n",
    "        # 初始化输入和目标 ID 列表\n",
    "        self.input_ids = []\n",
    "        self.target_ids = []\n",
    "\n",
    "        # 对整个文本进行分词\n",
    "        token_ids = tokenizer.encode(txt, allowed_special={'<|endoftext|>'})\n",
    "\n",
    "        # 使用滑动窗口将文本分割成重叠的 max_length 长度的序列\n",
    "        for i in range(0, len(token_ids) - max_length, stride):\n",
    "            input_chunk = token_ids[i:i + max_length]\n",
    "            target_chunk = token_ids[i + 1: i + max_length + 1]\n",
    "            # 将分词 ID 转换为 PyTorch 张量并添加到列表\n",
    "            self.input_ids.append(torch.tensor(input_chunk))\n",
    "            self.target_ids.append(torch.tensor(target_chunk))\n",
    "\n",
    "    def __len__(self):\n",
    "        # 返回数据集中的样本数量\n",
    "        return len(self.input_ids)\n",
    "\n",
    "    def __getitem__(self, idx):\n",
    "        # 根据索引返回对应的输入和目标张量\n",
    "        return self.input_ids[idx], self.target_ids[idx]\n",
    "\n",
    "# 创建数据加载器的函数\n",
    "def create_dataloader(txt, batch_size=4, max_length=256, stride=128, shuffle=True):\n",
    "    # 初始化分词器\n",
    "    tokenizer = tiktoken.get_encoding(\"gpt2\")\n",
    "\n",
    "    # 创建数据集\n",
    "    dataset = GPTDatasetV1(txt, tokenizer, max_length, stride)\n",
    "\n",
    "    # 创建数据加载器\n",
    "    dataloader = DataLoader(dataset, batch_size=batch_size, shuffle=shuffle)\n",
    "\n",
    "    return dataloader\n",
    "\n",
    "# 读取文本文件\n",
    "with open(\"small-text-sample.txt\", \"r\", encoding=\"utf-8\") as f:\n",
    "    raw_text = f.read()\n",
    "\n",
    "# 初始化分词器\n",
    "tokenizer = tiktoken.get_encoding(\"gpt2\")\n",
    "# 对原始文本进行编码\n",
    "encoded_text = tokenizer.encode(raw_text)\n",
    "\n",
    "# 定义词汇表大小、输出维度、最大长度和块大小\n",
    "vocab_size = 50257\n",
    "output_dim = 256\n",
    "max_len = 1024\n",
    "block_size = max_len\n",
    "\n",
    "# 创建词嵌入层和位置嵌入层\n",
    "token_embedding_layer = nn.Embedding(vocab_size, output_dim)\n",
    "pos_embedding_layer = torch.nn.Embedding(block_size, output_dim)\n",
    "\n",
    "# 设置最大长度为 4，这可能是一个错误，因为通常最大长度会大于 4\n",
    "max_length = 4\n",
    "# 创建数据加载器\n",
    "dataloader = create_dataloader(raw_text, batch_size=8, max_length=max_length, stride=5)"
   ],
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     "start_time": "2024-02-28T10:24:28.599202800Z"
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   },
   "id": "d6020b82c7b6dd6b"
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  {
   "cell_type": "code",
   "execution_count": 2,
   "id": "664397bc-6daa-4b88-90aa-e8fc1fbd5846",
   "metadata": {
    "ExecuteTime": {
     "end_time": "2024-02-28T10:24:30.875711300Z",
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   "outputs": [],
   "source": [
    "# 遍历数据加载器中的每个批次\n",
    "for batch in dataloader:\n",
    "    # 从当前批次中解包输入和目标数据\n",
    "    x, y = batch\n",
    "\n",
    "    # 使用词嵌入层计算输入序列的词嵌入\n",
    "    token_embeddings = token_embedding_layer(x)\n",
    "    # 使用位置嵌入层计算位置嵌入，这里使用 torch.arange 创建一个与 max_length 相同长度的序列\n",
    "    pos_embeddings = pos_embedding_layer(torch.arange(max_length))\n",
    "\n",
    "    # 将词嵌入和位置嵌入相加，得到最终的输入嵌入\n",
    "    input_embeddings = token_embeddings + pos_embeddings\n",
    "\n",
    "    # 跳出循环，这里可能是为了演示目的，实际训练中通常会继续循环直到遍历完所有数据\n",
    "    break"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "id": "d3664332-e6bb-447e-8b96-203aafde8b24",
   "metadata": {
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     "end_time": "2024-02-28T10:24:30.921720700Z",
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "torch.Size([8, 4, 256])\n"
     ]
    }
   ],
   "source": [
    "print(input_embeddings.shape)"
   ]
  },
  {
   "cell_type": "markdown",
   "source": [
    "# 第三章中的多头注意力"
   ],
   "metadata": {
    "collapsed": false
   },
   "id": "a6589588dfab8a73"
  },
  {
   "cell_type": "markdown",
   "source": [
    "## 方式A：简单实现"
   ],
   "metadata": {
    "collapsed": false
   },
   "id": "3960118f9f988847"
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "id": "a44e682d-1c3c-445d-85fa-b142f89f8503",
   "metadata": {
    "ExecuteTime": {
     "end_time": "2024-02-28T10:24:30.925722100Z",
     "start_time": "2024-02-28T10:24:30.891714900Z"
    }
   },
   "outputs": [],
   "source": [
    "# 定义因果自注意力模块\n",
    "class CausalSelfAttention(nn.Module):\n",
    "    def __init__(self, d_in, d_out, block_size, dropout, qkv_bias=False):\n",
    "        # 调用父类构造函数\n",
    "        super().__init__()\n",
    "        # 输出维度\n",
    "        self.d_out = d_out\n",
    "        # 查询、键和值的线性变换层\n",
    "        self.W_query = nn.Linear(d_in, d_out, bias=qkv_bias)\n",
    "        self.W_key = nn.Linear(d_in, d_out, bias=qkv_bias)\n",
    "        self.W_value = nn.Linear(d_in, d_out, bias=qkv_bias)\n",
    "        # Dropout层，用于正则化\n",
    "        self.dropout = nn.Dropout(dropout)\n",
    "        # 注册一个缓冲区，用于存储上三角掩码，用于因果自注意力\n",
    "        self.register_buffer('mask', torch.triu(torch.ones(block_size, block_size), diagonal=1))\n",
    "\n",
    "    def forward(self, x):\n",
    "        # 获取输入张量的批次大小、序列长度和输入维度\n",
    "        b, n_tokens, d_in = x.shape\n",
    "        # 分别计算查询、键和值\n",
    "        keys = self.W_key(x)\n",
    "        queries = self.W_query(x)\n",
    "        values = self.W_value(x)\n",
    "\n",
    "        # 计算注意力分数，这里使用了转置操作\n",
    "        attn_scores = queries @ keys.transpose(1, 2)\n",
    "        # 使用掩码将未来位置的注意力分数置为负无穷，实现因果自注意力\n",
    "        attn_scores.masked_fill_(self.mask.bool()[:n_tokens, :n_tokens], -torch.inf)\n",
    "        # 归一化注意力分数\n",
    "        attn_weights = torch.softmax(attn_scores / keys.shape[-1]**0.5, dim=1)\n",
    "        # 应用dropout\n",
    "        attn_weights = self.dropout(attn_weights)\n",
    "\n",
    "        # 计算上下文向量\n",
    "        context_vec = attn_weights @ values\n",
    "        return context_vec\n",
    "\n",
    "# 定义多头注意力包装器模块\n",
    "class MultiHeadAttentionWrapper(nn.Module):\n",
    "    def __init__(self, d_in, d_out, block_size, dropout, num_heads, qkv_bias=False):\n",
    "        # 调用父类构造函数\n",
    "        super().__init__()\n",
    "        # 创建多个因果自注意力模块\n",
    "        self.heads = nn.ModuleList([\n",
    "            CausalSelfAttention(d_in, d_out, block_size, dropout, qkv_bias) \n",
    "            for _ in range(num_heads)\n",
    "        ])\n",
    "        # 输出投影层，用于将多头的输出合并\n",
    "        self.out_proj = nn.Linear(d_out*num_heads, d_out*num_heads)\n",
    "\n",
    "    def forward(self, x):\n",
    "        # 将所有头的输出沿着最后一个维度拼接\n",
    "        context_vec = torch.cat([head(x) for head in self.heads], dim=-1)\n",
    "        # 通过输出投影层\n",
    "        return self.out_proj(context_vec)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "id": "7898551e-f582-48ac-9f66-3632abe2a93f",
   "metadata": {
    "ExecuteTime": {
     "end_time": "2024-02-28T10:24:30.927722100Z",
     "start_time": "2024-02-28T10:24:30.910719200Z"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "context_vecs.shape: torch.Size([8, 4, 256])\n"
     ]
    }
   ],
   "source": [
    "# 设置随机种子以确保结果的可重复性\n",
    "torch.manual_seed(123)\n",
    "\n",
    "# 定义块大小，这里与最大长度相同\n",
    "block_size = max_length\n",
    "# 输入维度\n",
    "d_in = output_dim\n",
    "\n",
    "# 定义多头注意力中每个头的输出维度\n",
    "num_heads = 2\n",
    "# 输出维度是输入维度除以头数\n",
    "d_out = d_in // num_heads\n",
    "\n",
    "# 初始化多头注意力包装器\n",
    "mha = MultiHeadAttentionWrapper(d_in, d_out, block_size, 0.0, num_heads)\n",
    "\n",
    "# 假设 input_embeddings 是之前准备好的输入数据\n",
    "batch = input_embeddings\n",
    "# 使用多头注意力模块处理输入数据\n",
    "context_vecs = mha(batch)\n",
    "\n",
    "# 打印上下文向量的维度\n",
    "print(\"context_vecs.shape:\", context_vecs.shape)"
   ]
  },
  {
   "cell_type": "markdown",
   "source": [
    "## 方式B：替代实现"
   ],
   "metadata": {
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   },
   "id": "6d5ad940a95ee6a7"
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "id": "2773c09d-c136-4372-a2be-04b58d292842",
   "metadata": {
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     "end_time": "2024-02-28T10:24:30.939725500Z",
     "start_time": "2024-02-28T10:24:30.923721600Z"
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   },
   "outputs": [],
   "source": [
    "# 定义多头自注意力模块\n",
    "class MultiHeadAttention(nn.Module):\n",
    "    def __init__(self, d_in, d_out, block_size, dropout, num_heads, qkv_bias=False):\n",
    "        # 调用父类构造函数\n",
    "        super().__init__()\n",
    "        # 确保输出维度可以被头数整除\n",
    "        assert d_out % num_heads == 0, \"d_out must be divisible by n_heads\"\n",
    "\n",
    "        # 初始化模块的属性\n",
    "        self.d_out = d_out\n",
    "        self.num_heads = num_heads\n",
    "        self.head_dim = d_out // num_heads  # 计算每个头的维度\n",
    "        self.W_query = nn.Linear(d_in, d_out, bias=qkv_bias)  # 查询线性层\n",
    "        self.W_key = nn.Linear(d_in, d_out, bias=qkv_bias)    # 键线性层\n",
    "        self.W_value = nn.Linear(d_in, d_out, bias=qkv_bias)  # 值线性层\n",
    "        self.out_proj = nn.Linear(d_out, d_out)  # 输出投影层\n",
    "        self.dropout = nn.Dropout(dropout)  # Dropout层\n",
    "        # 注册一个缓冲区，用于存储上三角掩码，用于因果自注意力\n",
    "        self.register_buffer('mask', torch.triu(torch.ones(block_size, block_size), diagonal=1))\n",
    "\n",
    "    def forward(self, x):\n",
    "        # 获取输入张量的批次大小、序列长度和输入维度\n",
    "        b, num_tokens, d_in = x.shape\n",
    "\n",
    "        # 分别计算查询、键和值\n",
    "        keys = self.W_key(x)\n",
    "        queries = self.W_query(x)\n",
    "        values = self.W_value(x)\n",
    "\n",
    "        # 将矩阵按头数分割，并添加一个维度\n",
    "        keys = keys.view(b, num_tokens, self.num_heads, self.head_dim)\n",
    "        values = values.view(b, num_tokens, self.num_heads, self.head_dim)\n",
    "        queries = queries.view(b, num_tokens, self.num_heads, self.head_dim)\n",
    "\n",
    "        # 转置以匹配多头注意力的维度\n",
    "        keys = keys.transpose(1, 2)\n",
    "        queries = queries.transpose(1, 2)\n",
    "        values = values.transpose(1, 2)\n",
    "\n",
    "        # 计算缩放点积注意力分数，并应用因果掩码\n",
    "        attn_scores = queries @ keys.transpose(2, 3)  # 对每个头进行点积\n",
    "        # 将掩码截断到与序列长度相匹配，并转换为布尔值\n",
    "        mask_bool = self.mask.bool()[:num_tokens, :num_tokens]\n",
    "        # 扩展掩码以匹配维度\n",
    "        mask_unsqueezed = mask_bool.unsqueeze(0).unsqueeze(0)\n",
    "        # 使用扩展的掩码填充注意力分数\n",
    "        attn_scores.masked_fill_(mask_unsqueezed, -torch.inf)\n",
    "        \n",
    "        # 归一化注意力分数\n",
    "        attn_weights = torch.softmax(attn_scores / keys.shape[-1]**0.5, dim=-1)\n",
    "        attn_weights = self.dropout(attn_weights)\n",
    "\n",
    "        # 计算上下文向量\n",
    "        context_vec = (attn_weights @ values).transpose(1, 2)\n",
    "        \n",
    "        # 合并头的输出\n",
    "        context_vec = context_vec.contiguous().view(b, num_tokens, self.d_out)\n",
    "        # 可选的输出投影\n",
    "        context_vec = self.out_proj(context_vec)\n",
    "\n",
    "        return context_vec"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "id": "779fdd04-0152-4308-af08-840800a7f395",
   "metadata": {
    "ExecuteTime": {
     "end_time": "2024-02-28T10:24:30.984735500Z",
     "start_time": "2024-02-28T10:24:30.941725600Z"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "context_vecs.shape: torch.Size([8, 4, 256])\n"
     ]
    }
   ],
   "source": [
    "# 设置随机种子以确保结果的可重复性\n",
    "torch.manual_seed(123)\n",
    "\n",
    "# 定义块大小，这里与最大长度相同\n",
    "block_size = max_length\n",
    "# 输入和输出维度\n",
    "d_in = output_dim\n",
    "# 输出维度设置为与输入维度相同\n",
    "d_out = d_in\n",
    "\n",
    "# 初始化多头自注意力模块\n",
    "mha = MultiHeadAttention(d_in, d_out, block_size, dropout=0.0, num_heads=2)\n",
    "\n",
    "# 假设 input_embeddings 是之前准备好的输入数据\n",
    "batch = input_embeddings\n",
    "# 使用多头自注意力模块处理输入数据\n",
    "context_vecs = mha(batch)\n",
    "\n",
    "# 打印上下文向量的维度\n",
    "print(\"context_vecs.shape:\", context_vecs.shape)"
   ]
  }
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