[Model] factoring out MambaMixer out of Jamba

[mzusman]

Factor out Mamba from Jamba to provide broader support for other Mamba-based models and enable future integration of the Mamba cache mechanism into the vLLM engine, alongside the KV cache.

[github-actions]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. @mzusman please rebase it. https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork