[Core] Simplify mm processing cache

[DarkLight1337]

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Purpose

Refactor the code related to multimodal processing and serialization in preparation for moving processing cache from P0 to P1.

Key changes:

• Move MultiModalKwargs out of the inner _apply_hf_processor_text_mm into the more outer _apply_hf_processor and _cached_apply_hf_processor.
• Remove unnecessary hashing inside ProcessingCache class.
• Remove unnecessary wrapper classes ProcessingCacheOptionalItem and ProcessingCacheItem.
• Split up serialization logic into individual methods to be more readable.

Test Plan

Test Result

(Optional) Documentation Update

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[gemini-code-assist]

Code Review

This pull request refactors the multimodal processing cache to simplify its logic. The key changes include moving the MultiModalKwargs creation to a higher level, removing hashing logic from within the ProcessingCache class, and eliminating unnecessary wrapper classes. The changes are well-structured and achieve the goal of simplification. I've found one issue with a type hint that should be addressed.

[Isotr0py]

LGTM, thanks for simplifying!