[2/N][refactor] torchair deepseek mla backend refactor

[linfeng-yuan]

What this PR does / why we need it?

This PR move current unified mla backend to torchair folder and remove torchair-related code in attention/mla_v1.py (1.3k -> 0.9k).

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Running eager mode with mla backend, and torchair mode with code before 2445

• vLLM version: v0.10.0
• vLLM main: vllm-project/vllm@f571ff8

[gemini-code-assist]

Code Review

This pull request refactors the attention backend selection logic and introduces a new TorchAir MLA backend for DeepSeek models on Ascend NPUs. The refactoring in platform.py correctly handles the selection of different attention backends. The new implementation in vllm_ascend/torchair/torchair_mla.py adds the TorchAir-based MLA backend. I've found a critical issue in the decode path of this new implementation that needs to be addressed.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 79.62264% with 216 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.56%. Comparing base (2bb7e55) to head (0b5b5b2).
⚠️ Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
vllm_ascend/torchair/torchair_mla.py	61.70%	211 Missing ⚠️
vllm_ascend/attention/mla_v1.py	85.29%	5 Missing ⚠️

❌ Your patch check has failed because the patch coverage (79.62%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2459      +/-   ##
==========================================
+ Coverage   76.18%   77.56%   +1.37%     
==========================================
  Files         120      130      +10     
  Lines       13532    17149    +3617     
==========================================
+ Hits        10310    13302    +2992     
- Misses       3222     3847     +625     

Flag	Coverage Δ
unittests	77.56% <79.62%> (+1.37%)	⬆️

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[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[wangxiyuan]

test passed here: https://github.com/vllm-project/vllm-ascend/actions/runs/17105103268?pr=2459

[wangxiyuan]

CI failue doesn't relate to this PR