[Misc] Fix backward compatibility from #23030

[ywang96]

Purpose

#23030 broke a v0 compatibility with mm embeddings as input and therefore the multimodal test. This PR fixes it.

Test Plan

pytest -v -s --ignore models/multimodal/generation/test_whisper.py --ignore models/multimodal/test_tensor_schema.py models/multimodal -m core_model

Test Result

(Optional) Documentation Update

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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.

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[gemini-code-assist]

Code Review

This pull request aims to fix a backward compatibility issue with multi-modal embeddings. The change modifies a return value in vllm/multimodal/base.py. However, this change introduces a type inconsistency within the from_seq_group function, where one return path now provides a dict while another provides a MultiModalKwargs object. This violates the function's type hint and can lead to runtime errors. This should be addressed to ensure consistent behavior.

[DarkLight1337]

Thanks for fixing

[ywang96]

All CI has passed in 5ea6a70. Force merging since all the following PRs only address pre-commit and doc build