[Multimodal] Simplify MM input definitions (#33331)

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

tests/v1/test_serial_utils.py

@@ -104,14 +104,10 @@ class MyRequest(msgspec.Struct):
 
 def test_multimodal_kwargs():
     e1 = MultiModalFieldElem(
-        "audio",
-        "a0",
         torch.zeros(1000, dtype=torch.bfloat16),
         MultiModalBatchedField(),
     )
     e2 = MultiModalFieldElem(
-        "video",
-        "v0",
         [torch.zeros(1000, dtype=torch.int8) for _ in range(4)],
         MultiModalFlatField(
             slices=[[slice(1, 2, 3), slice(4, 5, 6)], [slice(None, 2)]],
@@ -119,21 +115,20 @@ def test_multimodal_kwargs():
         ),
     )
     e3 = MultiModalFieldElem(
-        "image",
-        "i0",
         torch.zeros(1000, dtype=torch.int32),
         MultiModalSharedField(batch_size=4),
     )
     e4 = MultiModalFieldElem(
-        "image",
-        "i1",
         torch.zeros(1000, dtype=torch.int32),
         MultiModalFlatField(slices=[slice(1, 2, 3), slice(4, 5, 6)], dim=2),
     )
-    audio = MultiModalKwargsItem.from_elems([e1])
-    video = MultiModalKwargsItem.from_elems([e2])
-    image = MultiModalKwargsItem.from_elems([e3, e4])
-    mm = MultiModalKwargsItems.from_seq([audio, video, image])
+    mm = MultiModalKwargsItems(
+        {
+            "audio": [MultiModalKwargsItem({"a0": e1})],
+            "video": [MultiModalKwargsItem({"v0": e2})],
+            "image": [MultiModalKwargsItem({"i0": e3, "i1": e4})],
+        }
+    )
 
     # pack mm kwargs into a mock request so that it can be decoded properly
     req = MyRequest([mm])
@@ -147,8 +142,8 @@ def test_multimodal_kwargs():
 
     total_len = sum(memoryview(x).cast("B").nbytes for x in encoded)
 
-    # expected total encoding length, should be 14395, +-20 for minor changes
-    assert 14375 <= total_len <= 14425
+    # expected total encoding length, should be 14319, +-20 for minor changes
+    assert 14300 <= total_len <= 14340
     decoded = decoder.decode(encoded).mm[0]
     assert isinstance(decoded, MultiModalKwargsItems)