gpt.py

# Copyright (c) 2022, Tri Dao.

import math
from functools import partial

from collections import namedtuple
from collections.abc import Sequence

import torch
import torch.nn as nn
import torch.nn.functional as F

from transformers.models.gpt2.configuration_gpt2 import GPT2Config

from flash_attn.modules.mha import MHA
from flash_attn.modules.mlp import Mlp, FusedDenseGeluDense
from flash_attn.modules.block import Block
from flash_attn.modules.embedding import GPT2Embeddings

try:
    from flash_attn.ops.layer_norm import dropout_add_layer_norm
except ImportError:
    dropout_add_layer_norm = None

try:
    from flash_attn.ops.triton.mlp import FusedDenseSqreluDense
except ImportError:
    FusedDenseSqreluDense = None


def create_mixer_cls(config, layer_idx=None):
    head_dim = getattr(config, 'head_dim', config.hidden_size // config.num_attention_heads)
    softmax_scale = 1.0 if not config.scale_attn_weights else head_dim ** (-0.5)
    if config.scale_attn_by_inverse_layer_idx:
        assert layer_idx is not None
        softmax_scale /= float(layer_idx + 1)
    dwconv = getattr(config, 'attn_dwconv', False)
    rotary_emb_dim = int(getattr(config, 'rotary_emb_fraction', 0.0) * head_dim)
    use_flash_attn = getattr(config, 'use_flash_attn', False)
    fused_bias_fc = getattr(config, 'fused_bias_fc', False)
    mixer_cls = partial(MHA, num_heads=config.num_attention_heads, dropout=config.attn_pdrop,
                        softmax_scale=softmax_scale, causal=True, dwconv=dwconv,
                        rotary_emb_dim=rotary_emb_dim,
                        fused_bias_fc=fused_bias_fc, use_flash_attn=use_flash_attn)
    return mixer_cls


def create_mlp_cls(config, layer_idx=None):
    inner_dim = config.n_inner if config.n_inner is not None else 4 * config.hidden_size
    fused_dense_gelu_dense = getattr(config, 'fused_dense_gelu_dense', False)
    fused_dense_sqrelu_dense = getattr(config, 'fused_dense_sqrelu_dense', False)
    assert not (fused_dense_sqrelu_dense and fused_dense_gelu_dense)
    if not fused_dense_gelu_dense and not fused_dense_sqrelu_dense:
        mlp_cls = partial(Mlp, hidden_features=inner_dim,
                          activation=partial(F.gelu, approximate='tanh'))
    else:
        mlp_checkpoint_lvl = getattr(config, 'mlp_checkpoint_lvl', 0)
        # mlp_checkpoint_lvl could be a list, which contains the checkpoint_lvl for each layer
        if isinstance(mlp_checkpoint_lvl, Sequence):
            assert layer_idx is not None
            mlp_checkpoint_lvl = mlp_checkpoint_lvl[layer_idx]
        if fused_dense_gelu_dense:
            mlp_cls = partial(FusedDenseGeluDense, hidden_features=inner_dim,
                              checkpoint_lvl=mlp_checkpoint_lvl)
        elif fused_dense_sqrelu_dense:
            assert FusedDenseSqreluDense is not None
            mlp_cls = partial(FusedDenseSqreluDense, hidden_features=inner_dim,
                              checkpoint_lvl=mlp_checkpoint_lvl)
        else:
            raise RuntimeError('MLP type not supported')
    return mlp_cls


def create_block(config, layer_idx=None):
    mixer_cls = create_mixer_cls(config, layer_idx)
    mlp_cls = create_mlp_cls(config, layer_idx)
    norm_cls = partial(nn.LayerNorm, eps=config.layer_norm_epsilon)
    block = Block(config.hidden_size, mixer_cls, mlp_cls, norm_cls=norm_cls,
                  prenorm=True, resid_dropout=config.resid_pdrop,
                  fused_dropout_add_ln=getattr(config, 'fused_dropout_add_ln', False))
    block.layer_idx = layer_idx
    return block


# https://github.com/huggingface/transformers/blob/c28d04e9e252a1a099944e325685f14d242ecdcd/src/transformers/models/gpt2/modeling_gpt2.py#L454
def _init_weights(module, n_layer, initializer_range=0.02, rescale_prenorm_residual=True):
    if isinstance(module, nn.Linear):
        nn.init.normal_(module.weight, std=initializer_range)
        if module.bias is not None:
            nn.init.zeros_(module.bias)
    elif isinstance(module, nn.Embedding):
        nn.init.normal_(module.weight, std=initializer_range)

    if rescale_prenorm_residual:
        # Reinitialize selected weights subject to the OpenAI GPT-2 Paper Scheme:
        #   > A modified initialization which accounts for the accumulation on the residual path with model depth. Scale
        #   > the weights of residual layers at initialization by a factor of 1/√N where N is the # of residual layers.
        #   >   -- GPT-2 :: https://openai.com/blog/better-language-models/
        #
        # Reference (Megatron-LM): https://github.com/NVIDIA/Megatron-LM/blob/main/megatron/model/gpt_model.py
        for name, p in module.named_parameters():
            if name in ["out_proj.weight", "fc2.weight"]:
                # Special Scaled Initialization --> There are 2 Layer Norms per Transformer Block
                nn.init.normal_(p, mean=0.0, std=initializer_range / math.sqrt(2 * n_layer))


class GPT2Model(nn.Module):

    def __init__(self, config: GPT2Config):
        super().__init__()
        self.pad_vocab_size_multiple_8 = getattr(config, 'pad_vocab_size_multiple_8', False)
        if self.pad_vocab_size_multiple_8:
            if config.vocab_size % 8 != 0:
                config.vocab_size += 8 - (config.vocab_size % 8)

        self.embeddings = GPT2Embeddings(config.hidden_size, config.vocab_size,
                                         config.max_position_embeddings)
        self.emb_drop = nn.Dropout(config.embd_pdrop)

        # We change the order of residual and layer norm:
        # Instead of LN -> Attn / MLP -> Dropout -> Add, we do:
        # Attn / MLP -> Dropout -> Add -> LN, returning both the residual branch (output of Add) and
        # the main branch (output of LN). The model definition is unchanged, but the mapping of the
        # nn.LayerNorm weights are changed.
        # This is for performance reason: we can fuse dropout + add + layer_norm.
        self.fused_dropout_add_ln = getattr(config, 'fused_dropout_add_ln', False)
        if self.fused_dropout_add_ln and dropout_add_layer_norm is None:
            raise ImportError('dropout_add_layer_norm is not installed')
        # self.ln_0 is the first layer norm in the model, while self.ln_f (in the pretrained weight)
        # is the final layer norm.
        self.ln_0 = nn.LayerNorm(config.hidden_size, eps=config.layer_norm_epsilon)

        self.layers = nn.ModuleList([create_block(config, layer_idx=i)
                                     for i in range(config.num_hidden_layers)])

        self.apply(partial(_init_weights, n_layer=config.num_hidden_layers,
                           initializer_range=config.initializer_range))

    def forward(self, input_ids, position_ids=None):
        hidden_states = self.embeddings(input_ids, position_ids=position_ids)
        # TD [2022-07-30]: Force residual in fp32, seems to make fp16 training more stable
        if not self.fused_dropout_add_ln:
            residual = self.emb_drop(hidden_states).float()
            hidden_states = self.ln_0(residual.to(dtype=self.ln_0.weight.dtype))
        else:
            hidden_states, residual = dropout_add_layer_norm(
                hidden_states, None, self.ln_0.weight, self.ln_0.bias,
                self.emb_drop.p if self.training else 0.0, self.ln_0.eps, prenorm=True,
                residual_in_fp32=True
            )
        for layer in self.layers:
            hidden_states, residual = layer(hidden_states, residual)
        return hidden_states


class GPT2LMHeadModel(nn.Module):

    def __init__(self, config: GPT2Config):
        super().__init__()
        self.transformer = GPT2Model(config)
        self.lm_head = nn.Linear(config.n_embd, config.vocab_size, bias=False)
        # Initialize weights and apply final processing
        self.apply(partial(_init_weights, n_layer=config.num_hidden_layers,
                           initializer_range=config.initializer_range))
        self.tie_weights()

    def tie_weights(self):
        self.lm_head.weight = self.transformer.embeddings.word_embeddings.weight

    def forward(self, input_ids, position_ids=None):
        hidden_states = self.transformer(input_ids, position_ids=position_ids)
        lm_logits = self.lm_head(hidden_states)
        CausalLMOutput = namedtuple('CausalLMOutput', ['logits'])
        return CausalLMOutput(logits=lm_logits)