注意力机制详解及代码

前言：

自回归解码器推理是 Transformer 模型的 一个严重瓶颈，因为在每个解码步骤中加 载解码器权重以及所有注意键和值会产生 内存带宽开销

下图为三种注意力机制的结构图和实验结果

MHA

多头注意力机制是Transformer模型中的核心组件。在其设计中，"多头"意味着该机制并不只计算一种注意力权重，而是并行计算多种权重，每种权重都从不同的“视角”捕获输入的不同信息。

• hidden_state经过线性层得到q、k、v
• q、k、v经过split后增加一个维度：num_heads
• q、k计算注意力分数score
• softmax对注意力分数进行归一化得到注意力权重attention_probs
• 使用注意力权重和值计算输出：output
• 对注意力输出进行拼接concat

import torch
from torch import nn
class MutiHeadAttention(torch.nn.Module):
    def __init__(self, hidden_size, num_heads):
        super(MutiHeadAttention, self).__init__()
        self.num_heads = num_heads
        self.head_dim = hidden_size // num_heads
        
        ## 初始化Q、K、V投影矩阵
        self.q_linear = nn.Linear(hidden_size, hidden_size)
        self.k_linear = nn.Linear(hidden_size, hidden_size)
        self.v_linear = nn.Linear(hidden_size, hidden_size)
        
        ## 输出线性层
        self.o_linear = nn.Linear(hidden_size, hidden_size)
        
    def forward(self, hidden_state, attention_mask=None):
        batch_size = hidden_state.size()[0]
        
        query = self.q_linear(hidden_state)
        key = self.k_linear(hidden_state)
        value = self.v_linear(hidden_state)
        
        query = self.split_head(query)
        key = self.split_head(key)
        value = self.split_head(value)
        
        ## 计算注意力分数
        attention_scores = torch.matmul(query, key.transpose(-1, -2)) / torch.sqrt(torch.tensor(self.head_dim))
        
        if attention_mask != None:
            attention_scores += attention_mask * -1e-9
        
        ## 对注意力分数进行归一化
        attention_probs = torch.softmax(attention_scores, dim=-1)
        
        output = torch.matmul(attention_probs, value)
        
        ## 对注意力输出进行拼接
        output = output.transpose(-1, -2).contiguous().view(batch_size, -1, self.head_dim * self.num_heads)
        
        output = self.o_linear(output)
        
        return output
 
        
    def split_head(self, x):
        batch_size = x.size()[0]
        return x.view(batch_size, -1, self.num_heads, self.head_dim).transpose(1,2)

MQA

多查询注意力（MQA）可能导致质量下降和训练不稳定，并且训练针对质量和推理优化的单独模型可能不可行。此外，虽然一些语言模型已经使用了多查询注意力，如PaLM但许多语言模型没有，包括公开可用的语言模型，如T5和LLaM.

• hidden_state经过线性层得到q、k、v
• q、k、v经过split后增加一个维度：num_heads(q = num_heads,k=1,v=1)。相当于多个query，即多查询。
• q、k计算注意力分数score
• softmax对注意力分数进行归一化得到注意力权重attention_probs
• 使用注意力权重和值计算输出：output
• 对注意力输出进行拼接concat

## 多查询注意力
import torch
from torch import nn
class MutiQueryAttention(torch.nn.Module):
    def __init__(self, hidden_size, num_heads):
        super(MutiQueryAttention, self).__init__()
        self.num_heads = num_heads
        self.head_dim = hidden_size // num_heads
        
        ## 初始化Q、K、V投影矩阵
        self.q_linear = nn.Linear(hidden_size, hidden_size)
        self.k_linear = nn.Linear(hidden_size, self.head_dim) ###
        self.v_linear = nn.Linear(hidden_size, self.head_dim) ###
        
        ## 输出线性层
        self.o_linear = nn.Linear(hidden_size, hidden_size)
        
    def forward(self, hidden_state, attention_mask=None):
        batch_size = hidden_state.size()[0]
        
        query = self.q_linear(hidden_state)
        key = self.k_linear(hidden_state)
        value = self.v_linear(hidden_state)
        
        query = self.split_head(query)
        key = self.split_head(key, 1)
        value = self.split_head(value, 1)
        
        ## 计算注意力分数
        attention_scores = torch.matmul(query, key.transpose(-1, -2)) / torch.sqrt(torch.tensor(self.head_dim))
        
        if attention_mask != None:
            attention_scores += attention_mask * -1e-9
        
        ## 对注意力分数进行归一化
        attention_probs = torch.softmax(attention_scores, dim=-1)
        
        output = torch.matmul(attention_probs, value)
        
        output = output.transpose(-1, -2).contiguous().view(batch_size, -1, self.head_dim * self.num_heads)
        
        output = self.o_linear(output)
        
        return output
        
        
        
        
    def split_head(self, x, head_num=None):
        
        batch_size = x.size()[0]
        
        if head_num == None:
            return x.view(batch_size, -1, self.num_heads, self.head_dim).transpose(1,2)
        else:
            return x.view(batch_size, -1, head_num, self.head_dim).transpose(1,2)
    
    

GQA

• 使用 5% 的原始预训练 计算将现有的多头语言模型检查点训 练到具有 MQA 的模型中
• 引入分组查询注意力 (GQA)，这是多 头语言模型的泛化。查询注意力，它使用中间,多于一个，少于查询头数量的键值头。
• 经过训练的GQA 实现了接近多头注意力 的质量，并且速度与 MQA 相当。

• hidden_state经过线性层得到q、k、v
• q、k、v经过split后增加一个维度：num_heads(q = num_heads,k=group_num,v=group_num)。相当于把多头分组了，比如原先有10个头，那就是10个query,分成5组，每组2个query,1个value,1个key。
• q、k计算注意力分数score
• softmax对注意力分数进行归一化得到注意力权重attention_probs
• 使用注意力权重和值计算输出：output
• 对注意力输出进行拼接concat

## 分组注意力查询
import torch
from torch import nn
class MutiGroupAttention(torch.nn.Module):
    def __init__(self, hidden_size, num_heads, group_num):
        super(MutiGroupAttention, self).__init__()
        self.num_heads = num_heads
        self.head_dim = hidden_size // num_heads
        self.group_num = group_num
        
        ## 初始化Q、K、V投影矩阵
        self.q_linear = nn.Linear(hidden_size, hidden_size)
        self.k_linear = nn.Linear(hidden_size, self.group_num * self.head_dim)
        self.v_linear = nn.Linear(hidden_size, self.group_num * self.head_dim)
        
        ## 输出线性层
        self.o_linear = nn.Linear(hidden_size, hidden_size)
        
    def forward(self, hidden_state, attention_mask=None):
        batch_size = hidden_state.size()[0]
        
        query = self.q_linear(hidden_state)
        key = self.k_linear(hidden_state)
        value = self.v_linear(hidden_state)
        
        query = self.split_head(query)
        key = self.split_head(key, self.group_num)
        value = self.split_head(value, self.group_num)
        
        ## 计算注意力分数
        attention_scores = torch.matmul(query, key.transpose(-1, -2)) / torch.sqrt(torch.tensor(self.head_dim))
        
        if attention_mask != None:
            attention_scores += attention_mask * -1e-9
        
        ## 对注意力分数进行归一化
        attention_probs = torch.softmax(attention_scores, dim=-1)
        
        output = torch.matmul(attention_probs, value)
        
        output = output.transpose(-1, -2).contiguous().view(batch_size, -1, self.head_dim * self.num_heads)
        
        output = self.o_linear(output)
        
        return output
        
        
        
        
    def split_head(self, x, group_num=None):
        
        batch_size,seq_len = x.size()[:2]
        
        if group_num == None:
            return x.view(batch_size, -1, self.num_heads, self.head_dim).transpose(1,2)
        else:
            x = x.view(batch_size, -1, group_num, self.head_dim).transpose(1,2)
            x = x[:, :, None, :, :].expand(batch_size, group_num, self.num_heads // group_num, seq_len, self.head_dim).reshape(batch_size, self.num_heads // group_num * group_num, seq_len, self.head_dim)
            return x