Add option for subclasses to convert model and tokenizer in hf checkp…

…ointer (#1121)

llmfoundry/callbacks/hf_checkpointer.py

@@ -12,7 +12,7 @@
 import time
 from multiprocessing.context import SpawnProcess
 from pathlib import Path
-from typing import Any, Dict, List, Optional, Sequence, Union
+from typing import Any, Dict, List, Optional, Sequence, Tuple, Union
 
 import torch
 import torch.nn as nn
@@ -273,6 +273,23 @@ def _all_child_processes_done(self) -> bool:
         dist.all_reduce(x, reduce_operation='MAX')
         return x.item() == 0
 
+    def transform_model_and_tokenizer(
+        self, model: PreTrainedModel, tokenizer: PreTrainedTokenizerBase
+    ) -> Tuple[PreTrainedModel, PreTrainedTokenizerBase]:
+        """Transform the model and tokenizer before saving.
+
+        This allows a subclass to modify the model and tokenizer before saving. The base class implementation will
+        make no modifications.
+
+        Args:
+            model (PreTrainedModel): The model to be transformed.
+            tokenizer (PreTrainedTokenizerBase): The tokenizer to be transformed.
+
+        Returns:
+            Tuple[PreTrainedModel, PreTrainedTokenizerBase]: The transformed model and tokenizer.
+        """
+        return model, tokenizer
+
     def _save_checkpoint(self, state: State, logger: Logger):
         del logger  # unused
 
@@ -405,6 +422,10 @@ def dtensor_to_tensor_hook(
             new_model_instance.load_state_dict(state_dict, assign=True)
             del state_dict
 
+            # Transform the model and tokenizer before saving
+            new_model_instance, original_tokenizer = self.transform_model_and_tokenizer(
+                new_model_instance, original_tokenizer)
+
             log.debug('Saving Hugging Face checkpoint to disk')
             new_model_instance.save_pretrained(temp_save_dir)
             if original_tokenizer is not None: