[Model] Define merge_by_field_config MM interface (#25676)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

vllm/model_executor/models/interfaces.py

@@ -64,6 +64,12 @@ class SupportsMultiModal(Protocol):
     `multimodal_config.mm_encoder_tp_mode="data"`.
     """
 
+    merge_by_field_config: ClassVar[bool] = False
+    """
+    A flag that indicates which implementation of
+    `vllm.multimodal.utils.group_mm_kwargs_by_modality` to use.
+    """
+
     @classmethod
     def get_placeholder_str(cls, modality: str, i: int) -> Optional[str]:
         """