Fix extra BOS token in front of response for some tokenizers (#1003)

llmfoundry/data/finetuning/tasks.py

@@ -195,6 +195,29 @@ def _slice_chat_formatted_example(
     return prompt, response
 
 
+def _tokenize_with_bos_removal(tokenizer: PreTrainedTokenizerBase, text: str,
+                               text_target: str) -> TokenizedExample:
+    """Tokenizes the prompt and response using the provided tokenizer.
+
+    Args:
+        tokenizer (PreTrainedTokenizerBase): The tokenizer to use for tokenization.
+        prompt (str): The prompt to tokenize.
+        response (str): The response to tokenize.
+
+    Returns:
+        TokenizedExample: The tokenized example.
+    """
+    tokenized_sample = tokenizer(text=text, text_target=text_target)
+
+    # Remove the BOS token from the start of the labels if it was automatically added
+    if hasattr(tokenizer, 'add_bos_token') and tokenizer.add_bos_token:
+        if tokenizer.bos_token_id is not None and tokenized_sample['labels'][
+                0] == tokenizer.bos_token_id:
+            tokenized_sample['labels'] = tokenized_sample['labels'][1:]
+
+    return tokenized_sample
+
+
 def _tokenize_chat_formatted_example(
         example: ChatFormattedDict,
         tokenizer: PreTrainedTokenizerBase) -> TokenizedExample:
@@ -246,7 +269,11 @@ def _tokenize_prompt_response_formatted_example(
             f'Unable to tokenize example because {response_key} was not a string. {example=}'
         )
 
-    return tokenizer(text=prompt, text_target=response)
+    return _tokenize_with_bos_removal(
+        tokenizer=tokenizer,
+        text=prompt,
+        text_target=response,
+    )
 
 
 def tokenize_formatted_example(

tests/data/test_template_tokenization.py

@@ -163,3 +163,30 @@ def test_tokenize_instruct_example_well_formed():
             tokenized_example = tokenize_formatted_example(example, tokenizer)
             assert 'input_ids' in tokenized_example
             assert 'labels' in tokenized_example
+
+
+def test_tokenize_no_labels_bos_pr():
+    # This tokenizer automatically adds bos tokens
+    tokenizer = transformers.AutoTokenizer.from_pretrained(
+        'mistralai/Mixtral-8x7B-v0.1')
+
+    example = {'prompt': 'prompt', 'response': 'response'}
+
+    assert tokenizer.add_bos_token == True
+
+    tokenized_example = tokenize_formatted_example(example, tokenizer)
+
+    assert len(tokenized_example['labels']) == 1
+    assert tokenized_example['labels'][0] != tokenizer.bos_token_id
+    assert tokenized_example['input_ids'][0] == tokenizer.bos_token_id
+
+    # This tokenizer does not have the add_bos_token attribute
+    tokenizer = transformers.AutoTokenizer.from_pretrained('mosaicml/mpt-7b')
+
+    assert not hasattr(tokenizer, 'add_bos_token')
+
+    tokenized_example = tokenize_formatted_example(example, tokenizer)
+
+    assert len(tokenized_example['labels']) == 1
+    assert tokenized_example['labels'][0] != tokenizer.bos_token_id
+    assert tokenized_example['input_ids'][0] != tokenizer.bos_token_id