[Model] Move multimodal_cpu_fields definition to field config (#30181)

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

tests/distributed/test_shm_storage.py

@@ -28,7 +28,7 @@ def _dummy_elem(modality: str, key: str, size: int):
         modality=modality,
         key=key,
         data=torch.empty((size,), dtype=torch.int8),
-        field=MultiModalSharedField(1),
+        field=MultiModalSharedField(batch_size=1),
     )