[Optim] Compute multimodal hash only once per item

[DarkLight1337]

Introduce ProcessingCache.get_item to simultaneously get the hash and the corresponding multi-modal item from the cache, so that we don't have to recompute the hash again in BaseMultiModalProcessor.apply.

Notes:

• I intentionally omitted debug logging in ProcessingCache.get_item to avoid merge conflict with [Metrics] Log multi-modal cache stats #16478 .
• I also factored out some steps in BaseMultiModalProcessor._cached_apply_hf_processor and EncDecMultiModalProcessor.apply to promote code reuse.
• The enable_sanity_checks parameter to BaseMultiModalProcessor.__init__ has been removed as it's not that useful.

[github-actions]

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[Isotr0py]

Overall LGTM!