Merge 7bd999ba02 into bc1fca39f3

Before, when initiating a batch generation, the first sampled token was broadcasted to all sequences. This change now expands the output logits after a single efficient prefill, allowing for the sampling of a unique starting token for each sequence in the batch.

nanochat/engine.py

@@ -218,6 +218,8 @@ class Engine:
         ids = torch.tensor([tokens], dtype=torch.long, device=device)
         logits = self.model.forward(ids, kv_cache=kv_cache_prefill)
         logits = logits[:, -1, :]
+        # expand the logits to be of size (num_samples, vocab_size)
+        logits = logits.expand(num_samples, -1)
         next_ids = sample_next_token(logits, rng, temperature, top_k)  # (B, 1)
         sampled_tokens = next_ids[:, 0].tolist()
 
@@ -248,8 +250,6 @@ class Engine:
             # Get sampled tokens - either from prefill or from forward pass
             if first_iteration:
                 # Use the tokens we already sampled from prefill
-                sampled_tokens = [sampled_tokens[0]] * num_samples  # Broadcast first token to all rows
-                # TODO: we should sample a token for each row instead of broadcasting
                 first_iteration = False
             else:
                 # Forward the model and get the next token for each row