[Core][Multimodal] Allow passing multi_modal_uuids as multimodal identifiers. (#23394)

Signed-off-by: Roger Wang <hey@rogerw.io>

docs/features/multimodal_inputs.md

@@ -13,6 +13,41 @@ To input multi-modal data, follow this schema in [vllm.inputs.PromptType][]:
 - `prompt`: The prompt should follow the format that is documented on HuggingFace.
 - `multi_modal_data`: This is a dictionary that follows the schema defined in [vllm.multimodal.inputs.MultiModalDataDict][].
 
+### Stable UUIDs for Caching (multi_modal_uuids)
+
+When using multi-modal inputs, vLLM normally hashes each media item by content to enable caching across requests. You can optionally pass `multi_modal_uuids` to provide your own stable IDs for each item so caching can reuse work across requests without rehashing the raw content.
+
+??? code
+
+    ```python
+    from vllm import LLM
+    from PIL import Image
+
+    # Qwen2.5-VL example with two images
+    llm = LLM(model="Qwen/Qwen2.5-VL-3B-Instruct")
+
+    prompt = "USER: <image><image>\nDescribe the differences.\nASSISTANT:"
+    img_a = Image.open("/path/to/a.jpg")
+    img_b = Image.open("/path/to/b.jpg")
+
+    outputs = llm.generate({
+        "prompt": prompt,
+        "multi_modal_data": {"image": [img_a, img_b]},
+        # Provide stable IDs for caching.
+        # Requirements (matched by this example):
+        #  - Include every modality present in multi_modal_data.
+        #  - For lists, provide the same number of entries.
+        #  - Use None to fall back to content hashing for that item.
+        "multi_modal_uuids": {"image": ["sku-1234-a", None]},
+    })
+
+    for o in outputs:
+        print(o.outputs[0].text)
+    ```
+
+!!! warning
+    If both multimodal processor caching and prefix caching are disabled, user-provided `multi_modal_uuids` are ignored.
+
 ### Image Inputs
 
 You can pass a single image to the `'image'` field of the multi-modal dictionary, as shown in the following examples: