feat(mllm-chat): add mllm-chat submodule

[chenghuaWang]

Added mllm-chat as a new submodule with path mllm-chat and url
https://github.com/UbiquitousLearning/mllm-chat. Set update policy to none.

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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