[Model] Support Mamba2 (Codestral Mamba)

[tlrmchlsmth]

Add support for Mamba2. Not thoroughly tested yet, but Codestral Mamba has legible outputs.

Todo:

• Integration tests
• Support Chunked Prefill
• Incorporate mamba_chunk_scan_combined kernel to avoid the dependency on mamba_ssm
• Fix tensor parallelism
• Try to refactor the code for Mamba2's mixer layer to look more like Mamba's

Closes #6479

[github-actions]

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[tlrmchlsmth]

Notes on current state:

1. Now that [Kernel][Model] Improve continuous batching for Jamba and Mamba #9189 has landed, need to update the mamba_chunk_scan_combined to take cache indices, so that this PR will work with the updated MambaCacheManager. Until then this PR is not compatible with current main.
2. TP does seem to work in the present state however I see bad output when using CUDA graphs + custom_all_reduce