[VLM] Support caching in merged multi-modal processor

[DarkLight1337]

Compared to #11341, this PR moves the caching-related logic (e.g. checking if image is in the cache, and merging it with the processed outputs) outside of the main loop of applying the HF processor.

To enable this, the merged multi-modal processor for each model needs to define the "schema" of the output MultiModalKwargs (in BaseMultiModelProcessor._get_mm_fields_config), which defines how to obtain the kwargs corresponding to each item (MultiModalField.build_items) from the HF outputs, and also how to merge the kwargs of newly processed items with the cached results (MultiModalField.reduce).

[github-actions]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @DarkLight1337.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[DarkLight1337]

We now use HF's LlavaProcessor + our own prompt replacements to replicate the logic of MLlavaProcessor, so users don't have to install their GitHub anymore.