[Bugfix][Model] Add SupportsQuant interface to Mixtral

[tamas-czene-htec]

Encountered output changes with mixtral model after vllm update, I tracked down the root cause and the bug came in with the following PR: #12634

Without the following fix the qkv_proj is not loaded properly for quantized mixtral.

[github-actions]

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[mgoin]

cc @kylesayrs @dsikka

[github-actions]

This pull request has been automatically marked as stale because it has not had any activity within 90 days. It will be automatically closed if no further activity occurs within 30 days. Leave a comment if you feel this pull request should remain open. Thank you!

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @tamas-czene-htec.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[github-actions]

This pull request has been automatically marked as stale because it has not had any activity within 90 days. It will be automatically closed if no further activity occurs within 30 days. Leave a comment if you feel this pull request should remain open. Thank you!