Modify validation check for multimodal messages (#1787)

Co-authored-by: Daniel King <daniel@mosaicml.com>
Co-authored-by: Daniel King <43149077+dakinggg@users.noreply.github.com>

Author: 3 people

llmfoundry/data/finetuning/tasks.py

@@ -210,7 +210,8 @@ def _validate_chat_formatted_example(example: ChatFormattedDict):
             raise IncorrectMessageKeyQuantityError(list(message.keys()))
         if message[role_key] not in _ALLOWED_ROLES:
             raise InvalidRoleError(message[role_key], _ALLOWED_ROLES)
-        if not isinstance(message[content_key], str):
+        if not isinstance(message[content_key], (list, str)):
+            # this validation check accepts text-only as well as multimodal messages for chat-enabled models
             raise InvalidContentTypeError(type(message[content_key]))
         if last_message_role is not None and last_message_role == message[
             role_key]: