[Misc] Simplify max tokens in multimodal registry

[DarkLight1337]

Purpose

• get_max_tokens_per_item_by_nonzero_modality is redundant because get_max_tokens_per_item_by_modality already considers the limits by only passing in modalities with limit > 0.
• Don't create processor twice in get_max_tokens_per_item_by_modality.

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request correctly simplifies the logic for determining the maximum tokens for multimodal models by removing the redundant get_max_tokens_per_item_by_nonzero_modality function and avoiding the double creation of the multimodal processor within get_max_tokens_per_item_by_modality. The changes are clean and improve efficiency. However, I've identified a similar inefficiency in MultiModalBudget.__init__ where the processor is still created twice. Addressing this would further align with the goals of this PR.

[ywang96]

Looks like the one of the tests didn't pass cc @DarkLight1337

ValueError: At most 0 image(s) may be provided in one prompt.

[DarkLight1337]

Fixed in 10532e5