fix for token bug that skips EOS (#815)

This will actually add EOS if the tokenizer doesn't have it (HF doesn't do this)

src/levanter/data/text.py

@@ -929,15 +929,24 @@ def get_shard_source(self, split: str) -> Optional[ShardedDataSource[dict]]:
         )
 
 
-def preprocess_chat_example(batch, tokenizer: PreTrainedTokenizerBase) -> dict:
+def preprocess_chat_example(batch, tokenizer: PreTrainedTokenizerBase, should_append_eos: bool) -> dict:
     """
     Preprocess chat examples to match the format of preprocess_supervised_example.
     Returns a dict with input_ids and sources_len like the supervised case.
+
+    Args:
+        batch: List of dicts with input/output pairs
+        tokenizer: HuggingFace tokenizer
+        should_append_eos: Whether we need to manually add EOS (True if tokenizer doesn't do it automatically)
     """
     # Get sources (inputs) and targets (outputs) from the batch
     sources = [example["input"] for example in batch]
     targets = [example["output"] for example in batch]
 
+    # Add EOS only if needed (tokenizer doesn't do it automatically)
+    if should_append_eos:
+        targets = [t + tokenizer.eos_token for t in targets]
+
     # Tokenize sources alone first to get the source lengths
     sources_tokenized = tokenizer(sources, padding=False, truncation=True)
 
@@ -965,9 +974,13 @@ def mk_chat_sft_dataset(
     # Set up example structure matching supervised case
     output_exemplar = {"input_ids": np.zeros((0,), dtype=np.int32), "sources_len": np.zeros((0,), dtype=np.int32)}
 
+    input_ids = tokenizer("hi there")["input_ids"]
+    should_append_eos = input_ids[-1] != tokenizer.eos_token_id
+    logger.info(f"Manual EOS Needed: {should_append_eos}")
+
     # Process the dataset
     dataset = source.map_batches(
-        lambda ex: preprocess_chat_example(ex, tokenizer),
+        lambda ex: preprocess_chat_example(ex, tokenizer, should_append_eos),
         batch_size=128,
         num_cpus=num_cpus_used_by_tokenizer(tokenizer),
         output_exemplar=output_exemplar,