Simple change to enable mapping functions for ft constructor (#1468)

llmfoundry/data/finetuning/tasks.py

@@ -801,6 +801,7 @@ def build_from_hf(
         split: str,
         safe_load: bool = False,
         max_seq_len: int = 2048,
+        mapping_fn: Callable = tokenize_formatted_example,
         preprocessing_fn: Optional[Callable[[dict[str, Any]], Example]] = None,
         tokenizer: Optional[PreTrainedTokenizerBase] = None,
         target_prompts: str = DEFAULT_TARGET_PROMPTS,
@@ -824,6 +825,8 @@ def build_from_hf(
             max_seq_len (int): The maximum length of sequences
                 in the batch. See :class:`Seq2SeqFinetuningCollator` docstring
                 for details.
+            mapping_fn (Callable): The mapping function to use for mapping the data
+                examples.
             preprocessing_fn (Callable, optional): The preprocessing function to use for
                 formatting the data examples.
             tokenizer (PreTrainedTokenizerBase): The tokenizer to be used for tokenizing
@@ -930,11 +933,11 @@ def build_from_hf(
 
             def dataset_mapper(example: dict):
                 if preprocessing_fn is not None:
-                    return tokenize_formatted_example(
+                    return mapping_fn(
                         preprocessing_fn(example),
                         tokenizer,
                     )
-                return tokenize_formatted_example(example, tokenizer)
+                return mapping_fn(example, tokenizer)
 
             detected_cpu_count = os.cpu_count() or 1
             detected_cpus_with_margin = detected_cpu_count - 8