[Model] Deprecate merge_by_field_config=False

[DarkLight1337]

Purpose

With all models migrated in #26149, we can begin the process of phasing out merge_by_field_config=False. After v0.12, we will switch the default value of SupportsMultiModal.merge_by_field_config to True, then later remove the merge_by_field_config argument from group_mm_kwargs_by_modality entirely.

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 introduces a deprecation warning for merge_by_field_config=False in group_mm_kwargs_by_modality. The changes are straightforward and align with the goal of phasing out the legacy multi-modal kwarg batching. I've pointed out a potential inconsistency in the deprecation timeline mentioned in the warning message versus the PR description, which could be confusing for users. Otherwise, the changes look good.

[gemini-code-assist]

The deprecation warning message on this line states that the legacy code will be removed "in v0.12". However, the pull request description outlines a more gradual deprecation plan: "After v0.12, we will switch the default value ... then later remove the merge_by_field_config argument...". This suggests the removal will happen in a version after 0.12 (e.g., v0.13 or later). To provide a clear and accurate deprecation path for users, the warning message should be consistent with the planned timeline. I recommend updating the version mentioned in the warning to avoid confusion.

Suggested change

	"will be removed in v0.12. Please update your model with "
	"will be removed in a future version. Please update your model with "