model.py

import torch
import torch.nn as nn
from torch.nn import functional as F
import math

class CasualSelfAttention(nn.Module):
    def __init__(self,config):
        super().__init__()

        self.c_attn = nn.Linear(config.n_embd, config.n_embd * 3)
        self.c_proj = nn.Linear(config.n_embd, config.n_embd)
        self.n_head = config.n_head
        self.n_embd = config.n_embd

        self.c_proj.NANOGPT_SCALE_INIT = 1
        
    def forward(self,x):
        B,T, C = x.size()
        qkv = self.c_attn(x)
        q,k,v = qkv.split(self.n_embd,dim=2)

        q = q.view(B,T, self.n_head, C // self.n_head).transpose(1,2)
        k = k.view(B,T, self.n_head, C // self.n_head).transpose(1,2)
        v = v.view(B,T, self.n_head, C // self.n_head).transpose(1,2)

        y = F.scaled_dot_product_attention(q,k,v,is_causal=True)
        y = y.transpose(1,2).contiguous().view(B,T,C)
        y = self.c_proj(y)
        return y
    
class MLP(nn.Module):
    def __init__(self,config):
        super().__init__()

        self.increase = nn.Linear(config.n_embd,config.n_embd * 4)
        self.gelu = nn.GELU(approximate='tanh')
        self.reduce = nn.Linear(config.n_embd * 4,config.n_embd)

    def forward(self,x):
        x = self.increase(x)
        x = self.gelu(x)
        x = self.reduce(x)
        return x
    

class Block(nn.Module):
    def __init__(self,config):
        super().__init__()

        self.sa = CasualSelfAttention(config)
        self.mlp = MLP(config)
        self.ln1 = nn.LayerNorm(config.n_embd)
        self.ln2 = nn.LayerNorm(config.n_embd)

    def forward(self,x):
        x = x + self.sa(self.ln1(x))
        x = x + self.mlp(self.ln2(x))
        return x
    


class GPT(nn.Module):
    def __init__(self,config):
        super().__init__()
        self.config = config

        self.transformer = nn.ModuleDict(dict(
            wte = nn.Embedding(config.vocab_size,config.n_embd),
            wpe = nn.Embedding(config.block_size,config.n_embd),
            h = nn.ModuleList([Block(config) for _ in range(config.n_layer)]),
            ln_f = nn.LayerNorm(config.n_embd)
        ))

        self.lm_head = nn.Linear(config.n_embd,config.vocab_size,bias=False)


        self.transformer.wte.weight = self.lm_head.weight

        self.apply(self._init_weights)

    #
    def _init_weights(self, module):
        if isinstance(module, nn.Linear):
            std = 0.02
            if hasattr(module, "NANOGPT_SCALE_INIT"):
                std *= (2 * self.config.n_layer) ** -0.5

            torch.nn.init.normal_(module.weight, mean=0.0, std=std)

            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,idx,targets=None):
        B,T = idx.size()
        
        pos = torch.arange(0,T,dtype=torch.long,device=idx.device)
        tok_emb = self.transformer.wte(idx)
        pos_emb = self.transformer.wpe(pos)

        x = tok_emb + pos_emb

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

        x = self.transformer.ln_f(x)
        logits = self.lm_head(x)

        loss = None
        if targets is not None:
            loss = F.cross_entropy(logits.view(-1,logits.size(-1)),targets.view(-1))
        
        return logits,loss
    
    @torch.no_grad()
    def generate(self, idx, max_new_tokens, temperature=1.0, top_k=None):
        """
        Take a conditioning sequence of indices idx (LongTensor of shape (b,t)) and complete
        the sequence max_new_tokens times, feeding the predictions back into the model each time.
        Most likely you'll want to make sure to be in model.eval() mode of operation for this.
        """
        for _ in range(max_new_tokens):
            # if the sequence context is growing too long we must crop it at block_size
            idx_cond = idx if idx.size(1) <= self.config.block_size else idx[:, -self.config.block_size:]
            # forward the model to get the logits for the index in the sequence
            logits, _ = self(idx_cond)
            # pluck the logits at the final step and scale by desired temperature
            logits = logits[:, -1, :] / temperature
            # optionally crop the logits to only the top k options
            if top_k is not None:
                v, _ = torch.topk(logits, min(top_k, logits.size(-1)))
                logits[logits < v[:, [-1]]] = -float('Inf')
            # apply softmax to convert logits to (normalized) probabilities
            probs = F.softmax(logits, dim=-1)
            # sample from the distribution
            idx_next = torch.multinomial(probs, num_samples=1)
            # append sampled index to the running sequence and continue
            idx = torch.cat((idx, idx_next), dim=1)

        return idx