[Multimodal] Generate mm_hash based on request metadata when caching is turned off

[ywang96]

Purpose

Since #23308 and #22711, mm_hash is a required information required by the engine core. This introduced a regression when users explicitly specify enable_prefix_caching=False and disable_mm_preprocessor_cache=True (now as mm_processor_cache_gb=0) since previously mm_data will not be hashed under this scenario but now they will.

This PR add mm_hash_overrides generated based on request_id, modality string and index of the item in the modality list as the identifier of the item so that hashing is no longer required since request_id is considered unique inside vLLM.

Test Plan

Test Result

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[DarkLight1337]

LGTM if tests pass