[Core] Store only the keys for multi-modal data in P0 (#22198)

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

tests/multimodal/test_cache.py

@@ -0,0 +1,51 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import pytest
+import torch
+
+from vllm.multimodal.cache import MultiModalCache, MultiModalCacheItemMetadata
+from vllm.multimodal.inputs import (MultiModalFieldElem, MultiModalKwargs,
+                                    MultiModalKwargsItem,
+                                    MultiModalSharedField)
+
+
+def _dummy_elem(modality: str, key: str, size: int):
+    return MultiModalFieldElem(
+        modality=modality,
+        key=key,
+        data=torch.empty((size, ), dtype=torch.int8),
+        field=MultiModalSharedField(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_kw(size_by_key_modality: dict[str, dict[str, int]]):
+    return MultiModalKwargs.from_items([
+        _dummy_item(modality, size_by_key)
+        for modality, size_by_key in size_by_key_modality.items()
+    ])
+
+
+# yapf: disable
+@pytest.mark.parametrize(
+    ("item", "expected_size"),
+    [
+        (_dummy_item("a", {"a1": 100}), 100),
+        (_dummy_item("a", {"a1": 100, "a2": 110}), 210),
+        (_dummy_kw({"a": {"a1": 100, "a2": 110}, "b": {"b1": 120, "b2": 130}}), 460),  # noqa: E501
+    ],
+)
+# yapf: enable
+def test_cache_item_size(item, expected_size):
+    cache = MultiModalCache.get_lru_cache(2048, type(item))
+
+    cache[""] = item
+    assert cache.currsize == expected_size
+
+    cache[""] = MultiModalCacheItemMetadata.wraps(item)
+    assert cache.currsize == expected_size