聊聊 从源码来看ChatGLM-6B的模型结构

基于ChatGLM-6B第一版，要注意还有ChatGLM2-6B以及ChatGLM3-6B

概述

ChatGLM是transformer架构的神经网络模型，因此从transformer结构入手，分析其源码结构。
transformer结构：

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位置编码

ChatGLM-6B的位置编码采用的旋转位置编码(RoPB)实现。其源码：

class RotaryEmbedding(torch.nn.Module):
    def __init__(self, dim, base=10000, precision=torch.half, learnable=False):
        super().__init__()
        inv_freq = 1. / (base ** (torch.arange(0, dim, 2).float() / dim))
        inv_freq = inv_freq.half()
        self.learnable = learnable
        if learnable:
            self.inv_freq = torch.nn.Parameter(inv_freq)
            self.max_seq_len_cached = None
        else:
            self.register_buffer('inv_freq', inv_freq)
            self.max_seq_len_cached = None
            self.cos_cached = None
            self.sin_cached = None
        self.precision = precision

    def _load_from_state_dict(self, state_dict, prefix, local_metadata, strict, missing_keys, unexpected_keys,
                              error_msgs):
        pass

    def forward(self, x, seq_dim=1, seq_len=None):
        if seq_len is None:
            seq_len = x.shape[seq_dim]
        if self.max_seq_len_cached is None or (seq_len > self.max_seq_len_cached):
            self.max_seq_len_cached = None if self.learnable else seq_len
            t = torch.arange(seq_len, device=x.device, dtype=self.inv_freq.dtype)
            freqs = torch.einsum('i,j->ij', t, self.inv_freq)
            # Different from paper, but it uses a different permutation in order to obtain the same calculation
            emb = torch.cat((freqs, freqs), dim=-1).to(x.device)
            if self.precision == torch.bfloat16:
                emb = emb.float()

            # [sx, 1 (b * np), hn]
            cos_cached = emb.cos()[:, None, :]
            sin_cached = emb.sin()[:, None, :]
            if self.precision == torch.bfloat16:
                cos_cached = cos_cached.bfloat16()
                sin_cached = sin_cached.bfloat16()
            if self.learnable:
                return cos_cached, sin_cached
            self.cos_cached, self.sin_cached = cos_cached, sin_cached
        return self.cos_cached[:seq_len, ...], self.sin_cached[:seq_len, ...]

    def _apply(self, fn):
        if self.cos_cached is not None:
            self.cos_cached = fn(self.cos_cached)
        if self.sin_cached is not None:
            self.sin_cached = fn(self.sin_cached)
        return super()._apply(fn)

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激活函数

ChatGLM-6B采用的激活函数是GeLU(高斯误差线性单元)，其源码：

@torch.jit.script
def gelu_impl(x):
    """OpenAI's gelu implementation."""
    return 0.5 * x * (1.0 + torch.tanh(0.7978845608028654 * x *
                                       (1.0 + 0.044715 * x * x)))


def gelu(x):
    return gelu_impl(x)

编码器-解码器(encoder-decoder)

接下来就是编码器解码器结构，如何抓住模型源头来分析？可以从transformers的API入手：

from transformers import AutoTokenizer, AutoModel
tokenizer = AutoTokenizer.from_pretrained("THUDM/chatglm-6b", trust_remote_code=True)
model = AutoModel.from_pretrained("THUDM/chatglm-6b", trust_remote_code=True).half().to("cuda:1").eval()

print(mode)

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输出：

ChatGLMForConditionalGeneration(
  (transformer): ChatGLMModel(
    (word_embeddings): Embedding(130528, 4096)
    (layers): ModuleList(
      (0-27): 28 x GLMBlock(
        (input_layernorm): LayerNorm((4096,), eps=1e-05, elementwise_affine=True)
        (attention): SelfAttention(
          (rotary_emb): RotaryEmbedding()
          (query_key_value): Linear(in_features=4096, out_features=12288, bias=True)
          (dense): Linear(in_features=4096, out_features=4096, bias=True)
        )
        (post_attention_layernorm): LayerNorm((4096,), eps=1e-05, elementwise_affine=True)
        (mlp): GLU(
          (dense_h_to_4h): Linear(in_features=4096, out_features=16384, bias=True)
          (dense_4h_to_h): Linear(in_features=16384, out_features=4096, bias=True)
        )
      )
    )
    (final_layernorm): LayerNorm((4096,), eps=1e-05, elementwise_affine=True)
  )
  (lm_head): Linear(in_features=4096, out_features=130528, bias=False)
)

从脑图的角度来梳理下其结构

其结构图表示如下：

将结构图与最开始的transformer结构图对比来看，两者还是比较符合的。
官方源码中标注了编码器与解码器是一体的，只需要配置参数即可切换为解码器。如下：

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