model.py

import math
import torch
import torch.nn as nn
from torch.nn import functional as F
from transformers import PreTrainedModel
from .config import GPTConfig


################################
###         Layers           ###
################################

class Rotary(torch.nn.Module):

    def __init__(self, dim, base=10000):
        super().__init__()
        inv_freq = 1.0 / (base ** (torch.arange(0, dim, 2).float() / dim))
        self.register_buffer("inv_freq", inv_freq)
        self.seq_len_cached = None
        self.cos_cached = None
        self.sin_cached = None

    def forward(self, x):
        seq_len = x.shape[1]
        if seq_len != self.seq_len_cached:
            self.seq_len_cached = seq_len
            t = torch.arange(seq_len, device=x.device).type_as(self.inv_freq)
            freqs = torch.outer(t, self.inv_freq).to(x.device)
            self.cos_cached = freqs.cos()
            self.sin_cached = freqs.sin()
        return self.cos_cached[None, :, None, :], self.sin_cached[None, :, None, :]

def apply_rotary_emb(x, cos, sin):
    assert x.ndim == 4 # multihead attention
    d = x.shape[3]//2
    x1 = x[..., :d]
    x2 = x[..., d:]
    y1 = x1 * cos + x2 * sin
    y2 = x1 * (-sin) + x2 * cos
    return torch.cat([y1, y2], 3)

def rmsnorm(x0, eps=1e-6):
    x = x0.float()
    x = x * torch.rsqrt(x.pow(2).mean(-1, keepdim=True) + eps)
    return x.type_as(x0)


class RMSNorm(nn.Module):
    """ Root Mean Square Normalization """
    def __init__(self, dim: int, weight: bool = False, bias: bool = False, eps: float = 1e-6):
        super().__init__()
        self.eps = eps
        
        if weight:
            self.weight = nn.Parameter(torch.ones(dim))
        else:
            self.register_parameter("weight", None)

        if bias:
            self.bias = nn.Parameter(torch.zeros(dim))
        else:
            self.register_parameter("bias", None)

    def _norm(self, x):
        return x * torch.rsqrt(x.pow(2).mean(-1, keepdim=True) + self.eps)

    def forward(self, x):
        output = self._norm(x.float()).type_as(x)
        if self.weight is not None:
            output = output * self.weight
        if self.bias is not None:
            output = output + self.bias
        return output


class CausalSelfAttention(nn.Module):

    def __init__(self, config):
        super().__init__()
        self.n_head = config.n_head
        self.n_embd = config.n_embd
        self.head_dim = self.n_embd // self.n_head
        assert self.n_embd % self.n_head == 0
        # key, query, value projections for all heads, but in a batch
        self.c_attn = nn.Linear(self.n_embd, 3 * self.n_embd, bias=False)
        # output projection
        self.c_proj = nn.Linear(self.n_embd, self.n_embd, bias=False)
        self.rotary = Rotary(self.head_dim)

    def forward(self, x):
        B, T, C = x.size() # batch size, sequence length, embedding dimensionality (n_embd)
        # calculate query, key, values for all heads in batch and move head forward to be the batch dim
        qkv = self.c_attn(x)
        q, k, v = qkv.split(self.n_embd, dim=2)
        k = k.view(B, T, self.n_head, self.head_dim)
        q = q.view(B, T, self.n_head, self.head_dim)
        v = v.view(B, T, self.n_head, self.head_dim)
        cos, sin = self.rotary(q)
        q = apply_rotary_emb(q, cos, sin)
        k = apply_rotary_emb(k, cos, sin)
        y = F.scaled_dot_product_attention(q.transpose(1, 2), k.transpose(1, 2), v.transpose(1, 2), is_causal=True)
        y = y.transpose(1, 2).contiguous().view(B, T, C) # re-assemble all head outputs side by side
        # output projection
        y = self.c_proj(y)
        return y

class RMSNorm(nn.Module):
    def __init__(self, dim, eps=1e-5):
        super().__init__()
        self.eps = eps
        self.weight = nn.Parameter(torch.ones(dim))

    def forward(self, x):
        norm = torch.norm(x, dim=-1, keepdim=True)
        return self.weight * x / (norm + self.eps)

class Block(nn.Module):

    def __init__(self, config):
        super().__init__()
        self.attn = CausalSelfAttention(config)
        self.mlp = MLP(config)
        self.attn_scale = (1 / (2 * config.n_layer)**0.5)

    def forward(self, x):
        x = x + self.attn_scale * self.attn(rmsnorm(x))
        x = x + self.mlp(rmsnorm(x))
        return x

class MLP(nn.Module):

    def __init__(self, config):
        super().__init__()
        self.c_fc    = nn.Linear(config.n_embd, 4 * config.n_embd, bias=config.bias)
        self.gelu    = nn.GELU()
        self.c_proj  = nn.Linear(4 * config.n_embd, config.n_embd, bias=config.bias)
        self.dropout = nn.Dropout(config.dropout)

    def forward(self, x):
        x = self.c_fc(x)
        x = self.gelu(x)
        x = self.c_proj(x)
        x = self.dropout(x)
        return x


################################
###          Model           ###
################################

class GPT(PreTrainedModel):
    config_class = GPTConfig

    def __init__(self, config):
        super().__init__(config)
        self.transformer = nn.ModuleDict(dict(
            wte=nn.Embedding(config.vocab_size, config.n_embd),
            drop=nn.Dropout(config.dropout),
            h=nn.ModuleList([Block(config) for _ in range(config.n_layer)]),
        ))
        self.lm_head = nn.Linear(config.n_embd, config.vocab_size, bias=False)

        self.apply(self._init_weights)

        # GPT-2 style scaled init
        for pn, p in self.named_parameters():
            if pn.endswith('c_proj.weight'):
                torch.nn.init.normal_(p, mean=0.0, std=0.02 / math.sqrt(2 * config.n_layer))

    def _init_weights(self, module):
        if isinstance(module, nn.Linear):
            torch.nn.init.normal_(module.weight, mean=0.0, std=0.02)
            if module.bias is not None:
                torch.nn.init.zeros_(module.bias)
        elif isinstance(module, nn.Embedding):
            torch.nn.init.normal_(module.weight, mean=0.0, std=0.02)

    def forward(self, input_ids, labels=None):
        tok_emb = self.transformer.wte(input_ids)
        x = self.transformer.drop(tok_emb)

        for block in self.transformer.h:
            x = block(x)
        x = rmsnorm(x)

        logits = self.lm_head(x)

        loss = None
        if labels is not None:
            loss = F.cross_entropy(logits.view(-1, logits.size(-1)), labels.view(-1), ignore_index=-1)

        return {'loss': loss, 'logits': logits} if loss is not None else {'logits': logits}

    @torch.no_grad()
    def generate(self, idx, max_new_tokens, temperature=1.0, top_k=None):
        for _ in range(max_new_tokens):
            idx_cond = idx if idx.size(1) <= self.config.block_size else idx[:, -self.config.block_size:]
            logits = self(idx_cond)['logits']
            logits = logits[:, -1, :] / temperature
            if top_k is not None:
                v, _ = torch.topk(logits, min(top_k, logits.size(-1)))
                logits[logits < v[:, [-1]]] = -float('Inf')
            probs = F.softmax(logits, dim=-1)
            idx_next = torch.multinomial(probs, num_samples=1)
            idx = torch.cat((idx, idx_next), dim=1)
        return idx