[Refactor] Allow optional MultiModalKwargsItem in IPC (#23022)

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

tests/v1/core/test_prefix_caching.py

@@ -9,9 +9,7 @@ import pytest
 import torch
 
 from vllm.distributed.kv_events import AllBlocksCleared, BlockRemoved
-from vllm.multimodal.inputs import (MultiModalBatchedField,
-                                    MultiModalFieldElem, MultiModalKwargsItem,
-                                    PlaceholderRange)
+from vllm.multimodal.inputs import MultiModalKwargsItem, PlaceholderRange
 from vllm.sampling_params import SamplingParams
 from vllm.utils import sha256, sha256_cbor_64bit
 from vllm.v1.core.block_pool import BlockPool
@@ -37,13 +35,7 @@ def make_request(
     if mm_positions is None:
         mm_kwargs = None
     else:
-        mm_elem = MultiModalFieldElem(
-            modality="dummy_m",
-            key="dummy_k",
-            data=None,
-            field=MultiModalBatchedField(),
-        )
-        mm_item = MultiModalKwargsItem.from_elems([mm_elem])
+        mm_item = MultiModalKwargsItem.dummy("dummy_m")
         mm_kwargs = [mm_item] * len(mm_positions)
 
     return Request(request_id=request_id,