[Multimodal] Simplify MM input definitions (#33331)

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

tests/distributed/test_shm_storage.py

@@ -23,18 +23,16 @@ from vllm.multimodal.inputs import (
 )
 
 
-def _dummy_elem(modality: str, key: str, size: int):
+def _dummy_elem(size: int):
     return MultiModalFieldElem(
-        modality=modality,
-        key=key,
         data=torch.empty((size,), dtype=torch.int8),
         field=MultiModalSharedField(batch_size=1),
     )
 
 
-def _dummy_item(modality: str, size_by_key: dict[str, int]):
-    return MultiModalKwargsItem.from_elems(
-        [_dummy_elem(modality, key, size) for key, size in size_by_key.items()]
+def _dummy_item(size_by_key: dict[str, int]):
+    return MultiModalKwargsItem(
+        {key: _dummy_elem(size) for key, size in size_by_key.items()}
     )
 
 
@@ -61,7 +59,7 @@ class TestSingleWriterShmObjectStorage(unittest.TestCase):
     def test_minimal_put_get_cycle(self):
         """Test basic put and get operations."""
         key = "test_key"
-        value = _dummy_item("text", {"field1": 10, "field2": 20})
+        value = _dummy_item({"field1": 10, "field2": 20})
 
         # Put operation
         address, monotonic_id = self.storage.put(key, value)