modeling_gpt1.py

""" PyTorch GPT1 model."""

import math

import torch
from torch import nn
from transformers import PreTrainedModel
from transformers.modeling_outputs import (
    BaseModelOutput,
    CausalLMOutput,
)
from transformers.activations import get_activation

from configuration_gpt1 import GPT1Config


class GPT1MLP(nn.Module):
    def __init__(self, config: GPT1Config):
        super().__init__()
        self.activation_fn = get_activation(config.hidden_act)
        self.fc1 = nn.Linear(config.hidden_size, config.intermediate_size)
        self.fc2 = nn.Linear(config.intermediate_size, config.hidden_size)

    def forward(self, hidden_state):
        hidden_state = self.fc1(hidden_state)
        hidden_state = self.activation_fn(hidden_state)
        hidden_state = self.fc2(hidden_state)
        return hidden_state


class GPT1Attention(nn.Module):
    def __init__(self, config: GPT1Config):
        """
        Multi-head attention layer.
        """
        super().__init__()

        assert config.hidden_size % config.num_attention_heads == 0
        self.hidden_size = config.hidden_size
        self.num_heads = config.num_attention_heads
        self.head_dim = self.hidden_size // self.num_heads
        self.attn_dropout = nn.Dropout(p=config.attention_dropout)

        self.q_proj = nn.Linear(self.hidden_size, self.num_heads * self.head_dim)
        self.k_proj = nn.Linear(self.hidden_size, self.num_heads * self.head_dim)
        self.v_proj = nn.Linear(self.hidden_size, self.num_heads * self.head_dim)
        self.o_proj = nn.Linear(self.num_heads * self.head_dim, self.hidden_size)

    def forward(self, hidden_state, attn_mask):
        bs, seq_len, _ = hidden_state.size() # (batch_size, seq_len, dim)

        # linearly project the inputs
        Q = self.q_proj(hidden_state) # (batch_size, seq_len, n_heads * head_dim)
        K = self.k_proj(hidden_state)
        V = self.v_proj(hidden_state)

        # split into n_heads to compute attention
        queries = Q.view(bs, seq_len, self.num_heads, self.head_dim).transpose(1, 2) # (batch_size, n_heads, seq_len, head_dim)
        keys = K.view(bs, seq_len, self.num_heads, self.head_dim).transpose(1, 2)
        values = V.view(bs, seq_len, self.num_heads, self.head_dim).transpose(1, 2)

        # compute attention matmul
        keys = keys.transpose(2, 3) # (batch_size, n_heads, head_dim, seq_len)
        attn_scores = queries @ keys # (batch_size, n_heads, seq_len, seq_len)

        # scale
        attn_scores = attn_scores / math.sqrt(self.head_dim)

        # mask
        if attn_mask is not None:
            attn_scores = attn_scores + attn_mask

        # softmax (attention probabilities) + dropout
        attn_probs = nn.functional.softmax(attn_scores, dim=-1, dtype=torch.float32).to(Q.dtype)
        attn_probs = self.attn_dropout(attn_probs)

        # matmul
        attn_output = attn_probs @ values # (batch_size, n_heads, seq_len, head_dim)

        attn_output = attn_output.transpose(1, 2).contiguous()
        attn_output = attn_output.reshape(bs, seq_len, self.hidden_size) # (batch_size, seq_len, n_heads * head_dim)

        # final linear
        attn_output = self.o_proj(attn_output)
        return attn_output


class GPT1DecoderLayer(nn.Module):
    def __init__(self, config: GPT1Config):
        super().__init__()
        self.attention = GPT1Attention(config)
        self.mlp = GPT1MLP(config)

        self.attention_norm = nn.LayerNorm(normalized_shape=config.hidden_size,
                                           eps=config.layer_norm_eps)
        self.mlp_norm = nn.LayerNorm(normalized_shape=config.hidden_size,
                                     eps=config.layer_norm_eps)

        self.res_dropout = nn.Dropout(p=config.resid_pdrop)

    def forward(self, hidden_state, attn_mask):
        # attention
        residual = hidden_state
        hidden_state = self.attention(hidden_state, attn_mask)
        hidden_state = self.res_dropout(hidden_state)
        hidden_state = residual + hidden_state
        hidden_state = self.attention_norm(hidden_state)

        # feed forward fully connected
        residual = hidden_state
        hidden_state = self.mlp(hidden_state)
        hidden_state = self.res_dropout(hidden_state)
        hidden_state = residual + hidden_state
        hidden_state = self.mlp_norm(hidden_state)

        return hidden_state


class GPT1PreTrainedModel(PreTrainedModel):
    config_class = GPT1Config
    supports_gradient_checkpointing = False

    def _init_weights(self, module):
        std = self.config.initializer_range
        if isinstance(module, nn.Linear):
            module.weight.data.normal_(mean=0.0, std=std)
            if module.bias is not None:
                module.bias.data.zero_()
        elif isinstance(module, nn.Embedding):
            module.weight.data.normal_(mean=0.0, std=std)
            if module.padding_idx is not None:
                module.weight.data[module.padding_idx].zero_()


class GPT1Model(GPT1PreTrainedModel):

    def __init__(self, config: GPT1Config):
        super().__init__(config)

        # embeddings
        self.embs = nn.Embedding(config.vocab_size, config.hidden_size)
        self.embs_dropout = nn.Dropout(p=config.embd_pdrop)

        # positional encoding (learned)
        self.pos_emb = nn.Embedding(config.max_position_embeddings,
                                    config.hidden_size)

        self.layers = nn.ModuleList(
            [GPT1DecoderLayer(config) for _ in range(config.num_hidden_layers)]
        )

        self.post_init()

    def get_input_embeddings(self):
        return self.embs

    def set_input_embeddings(self, value):
        self.embs = value

    def forward(self, input_ids, *args, **kwargs):
        position_ids = torch.arange(input_ids.size(-1),
                                    dtype=torch.long,
                                    device=input_ids.device).unsqueeze_(0)

        input_embeds = self.embs(input_ids) # (bs, seq_len, dim)
        position_embeds = self.pos_emb(position_ids)
        hidden_state = self.embs_dropout(input_embeds) + position_embeds

        seq_len = input_ids.size(-1)
        attn_mask = torch.full((seq_len, seq_len), fill_value=float('-inf'))
        attn_mask = torch.triu(attn_mask, diagonal=1)

        causal_mask = attn_mask.to(dtype=input_embeds.dtype,
                                   device=input_embeds.device)

        for layer in self.layers:
            hidden_state = layer(hidden_state, attn_mask=causal_mask)

        return BaseModelOutput(
            last_hidden_state=hidden_state
        )


class GPT1ForCausalLM(GPT1PreTrainedModel):
    _tied_weights_keys = ["lm_head.weight"]

    def __init__(self, config: GPT1Config):
        super().__init__(config)
        self.model = GPT1Model(config)
        self.vocab_size = config.vocab_size

        self.lm_head = nn.Linear(config.hidden_size, config.vocab_size)

        # initialize weigths and apply final processing
        self.post_init()

    def get_input_embeddings(self):
        return self.model.embs

    def set_input_embeddings(self, value):
        self.model.embs = value

    def get_output_embeddings(self):
        return self.lm_head

    def set_output_embeddings(self, new_embeddings):
        self.lm_head = new_embeddings

    def get_decoder(self):
        return self.model

    def set_decoder(self, decoder):
        self.model = decoder

    def forward(self, input_ids, labels=None, *args, **kwargs):
        output = self.model(input_ids)

        hidden_state = output[0]
        logits = self.lm_head(hidden_state).float()

        loss = None
        if labels is not None:
            shift_logits = logits[..., :-1, :].contiguous()
            shift_labels = labels[..., 1:].contiguous()

            loss_fn = torch.nn.CrossEntropyLoss()
            shift_logits = shift_logits.view(-1, self.config.vocab_size)
            shift_labels = shift_labels.view(-1)
            loss = loss_fn(shift_logits, shift_labels)

        return CausalLMOutput(
            loss=loss,
            logits=logits
        )

    def prepare_inputs_for_generation(self, input_ids, *args, **kwargs):
        return { 'input_ids': input_ids }