[MUSA][Feat] Upgrade MATE to match CUDA/ROCm behavior on FA

[yeahdongcn]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

Upgrade MATE to match CUDA/ROCm behavior on Flash Attention.

Test Plan

Regression tests.

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan. Please provide the test scripts & test commands. Please state the reasons if your codes don't require additional test scripts. For test file guidelines, please check the test style doc
• The test results. Please paste the results comparison before and after, or the e2e results.
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model. Please run mkdocs serve to sync the documentation editions to ./docs.
• (Optional) Release notes update. If your change is user-facing, please update the release notes draft.

BEFORE SUBMITTING, PLEASE READ https://github.com/vllm-project/vllm-omni/blob/main/CONTRIBUTING.md (anything written below this line will be removed by GitHub Actions)

[hsliuustc0106]

BLOCKER scan:

• Correctness: PASS
• Reliability/Safety: PASS
• Breaking Changes: PASS
• Test Coverage: PASS (MUSA CI should cover this)
• Documentation: PASS (internal backend change)
• Security: PASS

OVERALL: NO BLOCKERS

VERDICT: COMMENT

Note: The logic change in is_mate_available() looks correct - checking for either flash_attn_func or flash_attn_varlen_func is appropriate given the new flash_attn_interface import.

[gcanlin]

LGTM