[Attention] Unify mamba and attention backend selection

[ayushsatyam146]

Purpose

Fixes #23073. This PR unifies mamba and attention backend selection logic by removing the separate get_mamba_attn_backend() function and implementing the standard Layer.get_attn_backend() interface for all layer types.

Fixes the issue where mamba models used duplicate backend selection logic instead of the unified approach.

Changes Made:

• Deleted vllm/v1/attention/backends/mamba_selectors.py to eliminate duplicate code
• Updated MambaMixer and MambaMixer2 classes to implement get_attn_backend() method
• Modified gpu_model_runner.py to use the unified get_attn_backends_for_layers() function for all layer types
• Removed mamba-specific backend selection logic from model runner
• Updated affected model files to use the new unified interface

This makes mamba models more pluggable and follows the same pattern as regular attention layers:

• MambaMixer.get_attn_backend() → Mamba1AttentionBackend
• MambaMixer2.get_attn_backend() → Mamba2AttentionBackend

[github-actions]

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[gemini-code-assist]

Code Review

This pull request effectively unifies the attention backend selection logic for Mamba and standard attention layers, which is a great improvement for code maintainability and pluggability. The changes in the model runner and layer classes are well-implemented. My main feedback is on the testing side, where I've identified an opportunity to consolidate and improve the new tests for better clarity and correctness.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ayushsatyam146.

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

[Josephasafg]

I think this is a good and cleaner approach overall. Lets let @tdoublep / @heheda12345 give their opinion as well

[ayushsatyam146]

Hi @heheda12345 @LucasWilkinson @Josephasafg, this PR needed rebase because ShortConvAttentionBackend was added and my PR needed to accommodate it's changes as well. I have rebased it and addressed all concerns, including some new suggestions by @Josephasafg. Please review, Thanks!

[ayushsatyam146]

Hi @LucasWilkinson @heheda12345 @Josephasafg, this is a gentle reminder to take a look at this PR. I have added all the suggested changes and addressed all the concerns regarding this PR. Please let me know if anything else needs a change.

[heheda12345]

Thanks! Only a few small comments.

[ayushsatyam146]

Thanks @heheda12345 for reviewing again. I addressed your comments and there was another merge conflict that needed to be addressed so I fixed that too. PTAL when you get time, Thanks!
cc: @LucasWilkinson @Josephasafg @killershrimp

[heheda12345]

LGTM! Thanks for the clean-up.

[LucasWilkinson]

LGTM; thanks for doing this!

[ayushsatyam146]

The CI checks have failed @heheda12345 @LucasWilkinson and I am not sure if it is because of my changes or not. What should I do in this case? Am I supposed to patch a fix for this or is this a flaky CI check?