[Multimodal] Simplify MM input definitions

[DarkLight1337]

Purpose

Remove unnecessary fields in MultiModalFieldElem; instead, always pass dictionaries to construct MultiModalKwargsItem and MultiModalKwargsItems to make their roles clearer. As group_mm_kwargs_by_modality still requires modality information, it has been updated to receive a list of (modality, MultiModalKwargsItem) instead of just a list of MultiModalKwargsItem.

Also, improve related docstrings.

Test Plan

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[gemini-code-assist]

Code Review

This pull request refactors the multimodal input definitions to simplify them by removing redundant fields from MultiModalFieldElem. The changes are consistently applied across the codebase, including tests and serialization logic. The new design is cleaner and more maintainable. I found one critical bug in a test file where a tuple was constructed in the wrong order, which would lead to a runtime error. Otherwise, the changes look good.

[mergify]

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