[Build] Update vllm to point to vllm-project/flash-attention commit that builds FA3 with torch stable API.

[cleonard530]

Purpose

As a part of #26946. Updated vllm_flash_attn.cmake to use FA fork that builds a libtorch ABI stable FA3 library.

Currently, this is pointing towards my flash-attention fork but if this passes the CI, we will merge that fork with vllm-project/flash-attention, then we will point to the new tag.

see vllm-project/flash-attention#152 to see the commit it is pointing to.

cc @janeyx99 @Harry-Chen

Test Plan

build with new flash-attention commit
pytest tests/kernels/attention/test_flash_attn.py -k "3"

Test Result

Build succeded

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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, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

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Migration progress of vLLM using the Audit Python extension torch-abi-audit:

-- extensions --
    [STABLE  ] [abi3-ok               ] _C_stable_libtorch.abi3.so  (stable_shim=87, unstable=0)
    [UNSTABLE] [abi3-ok               ] _flashmla_C.abi3.so  (stable_shim=0, unstable=72)
    [UNSTABLE] [abi3-ok               ] _flashmla_extension_C.abi3.so  (stable_shim=0, unstable=68)
    [STABLE  ] [abi3-ok               ] _moe_C_stable_libtorch.abi3.so  (stable_shim=72, unstable=0)
    [NO-TORCH] [abi3-ok               ] cumem_allocator.abi3.so
    [NO-TORCH] [abi3-ok               ] spinloop.abi3.so
    [UNSTABLE] [uses-private-api      ] third_party/deep_gemm/_C.cpython-312-x86_64-linux-gnu.so  (stable_shim=0, unstable=57)
    [UNSTABLE] [abi3-ok               ] vllm_flash_attn/_vllm_fa2_C.abi3.so  (stable_shim=0, unstable=84)
    [STABLE  ] [abi3-ok               ] vllm_flash_attn/_vllm_fa3_C.abi3.so  (stable_shim=65, unstable=0)

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[Harry-Chen]

Triggered a CI pipeline to test building

[cleonard530]

Thanks @Harry-Chen. Non of these failures seem to be caused by the updates to FA3.

[Harry-Chen]

@cleonard530 It is merged! Could you please change the commit ref to the main repo? (and the commit title)

[cleonard530]

Thanks @Harry-Chen, just updated the GIT_TAG!

[Harry-Chen]

LGTM, thanks!

[cleonard530]

@Harry-Chen, I checked each of these failures and I don't think they were coming from this PR. Also, since the test passed before when the tag pointed to my flash-attention fork, I don't think the last change would of broken anything. Can you please rerun the failed test and try to merge this?