inference.py

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

# hyperparameters
block_size = 256 # what is the maximum context length for predictions?
n_embd = 384
n_head = 6
n_layer = 6
dropout = 0.2
device = 'cuda' if torch.cuda.is_available() else 'cpu'

class Head(nn.Module):
    """ one head of self-attention """

    def __init__(self, head_size):
        super().__init__()
        self.key = nn.Linear(n_embd, head_size, bias=False)
        self.query = nn.Linear(n_embd, head_size, bias=False)
        self.value = nn.Linear(n_embd, head_size, bias=False)
        self.register_buffer('tril', torch.tril(torch.ones(block_size, block_size)))
        self.dropout = nn.Dropout(dropout)

    def forward(self, x):
        B,T,C = x.shape
        k = self.key(x)   
        q = self.query(x) 
        wei = q @ k.transpose(-2,-1) * k.shape[-1]**-0.5
        wei = wei.masked_fill(self.tril[:T, :T] == 0, float('-inf'))
        wei = F.softmax(wei, dim=-1)
        wei = self.dropout(wei)
        v = self.value(x)
        out = wei @ v
        return out

class MultiHeadAttention(nn.Module):
    """ multiple heads of self-attention in parallel """

    def __init__(self, num_heads, head_size):
        super().__init__()
        self.heads = nn.ModuleList([Head(head_size) for _ in range(num_heads)])
        self.proj = nn.Linear(head_size * num_heads, n_embd)
        self.dropout = nn.Dropout(dropout)

    def forward(self, x):
        out = torch.cat([h(x) for h in self.heads], dim=-1)
        out = self.dropout(self.proj(out))
        return out

class FeedFoward(nn.Module):
    """ a simple linear layer followed by a non-linearity """

    def __init__(self, n_embd):
        super().__init__()
        self.net = nn.Sequential(
            nn.Linear(n_embd, 4 * n_embd),
            nn.ReLU(),
            nn.Linear(4 * n_embd, n_embd),
            nn.Dropout(dropout),
        )

    def forward(self, x):
        return self.net(x)

class Block(nn.Module):
    """ Transformer block: communication followed by computation """

    def __init__(self, n_embd, n_head):
        super().__init__()
        head_size = n_embd // n_head
        self.sa = MultiHeadAttention(n_head, head_size)
        self.ffwd = FeedFoward(n_embd)
        self.ln1 = nn.LayerNorm(n_embd)
        self.ln2 = nn.LayerNorm(n_embd)

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

class GPTLanguageModel(nn.Module):

    def __init__(self):
        super().__init__()
        # each token directly reads off the logits for the next token from a lookup table
        self.token_embedding_table = nn.Embedding(vocab_size, n_embd)
        self.position_embedding_table = nn.Embedding(block_size, n_embd)
        self.blocks = nn.Sequential(*[Block(n_embd, n_head=n_head) for _ in range(n_layer)])
        self.ln_f = nn.LayerNorm(n_embd) # final layer norm
        self.lm_head = nn.Linear(n_embd, vocab_size)
        self.apply(self._init_weights)

    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, idx, targets=None):
        B, T = idx.shape
        tok_emb = self.token_embedding_table(idx)
        pos_emb = self.position_embedding_table(torch.arange(T, device=device))
        x = tok_emb + pos_emb
        x = self.blocks(x)
        x = self.ln_f(x)
        logits = self.lm_head(x)

        if targets is None:
            loss = None
        else:
            B, T, C = logits.shape
            logits = logits.view(B*T, C)
            targets = targets.view(B*T)
            loss = F.cross_entropy(logits, targets)

        return logits, loss

    def generate(self, idx, max_new_tokens):
        # idx is (B, T) array of indices in the current context
        for _ in range(max_new_tokens):
            # crop idx to the last block_size tokens
            idx_cond = idx[:, -block_size:]
            # get the predictions
            logits, loss = self(idx_cond)
            # focus only on the last time step
            logits = logits[:, -1, :] # becomes (B, C)
            # apply softmax to get probabilities
            probs = F.softmax(logits, dim=-1) # (B, C)
            # sample from the distribution
            idx_next = torch.multinomial(probs, num_samples=1) # (B, 1)
            # append sampled index to the running sequence
            idx = torch.cat((idx, idx_next), dim=1) # (B, T+1)
        return idx

def load_model(model_path='gpt_model.pkl'):
    """Load the trained model and its configuration."""
    device = 'cuda' if torch.cuda.is_available() else 'cpu'
    
    # Load the saved model and config
    checkpoint = torch.load(model_path, map_location=device)
    
    # Get the vocabulary and config
    vocab = checkpoint['vocab']
    config = checkpoint['config']
    
    # Set the global vocab_size for the model
    global vocab_size
    vocab_size = vocab['vocab_size']
    
    # Create a new model instance with the saved config
    model = GPTLanguageModel()
    model.load_state_dict(checkpoint['model_state_dict'])
    model = model.to(device)
    model.eval()
    
    return model, vocab, config, device

def generate_text(prompt, model, vocab, config, device, max_new_tokens=500):
    """Generate text from a prompt."""
    # Encode the prompt
    encode = lambda s: [vocab['stoi'][c] for c in s]
    decode = lambda l: ''.join([vocab['itos'][i] for i in l])
    
    # Convert prompt to tensor
    if prompt:
        context = torch.tensor(encode(prompt), dtype=torch.long, device=device).unsqueeze(0)
    else:
        context = torch.zeros((1, 1), dtype=torch.long, device=device)
    
    # Generate text
    with torch.no_grad():
        output = model.generate(context, max_new_tokens=max_new_tokens)
        return decode(output[0].tolist())

if __name__ == "__main__":
    # Example usage
    model, vocab, config, device = load_model('gpt_model.pkl')
    
    # Generate text with a prompt
    prompt = "Once upon a time"
    generated_text = generate_text(prompt, model, vocab, config, device)
    print(generated_text)