[BugFix] Fix FA2 RuntimeError when sinks is provided

[LucasWilkinson]

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.

Purpose

fix

RuntimeError: _vllm_fa2_C::varlen_fwd() expected at most 21 argument(s) but received 22 argument(s). Declaration: _vllm_fa2_C::varlen_fwd(Tensor($0! -> ) q, Tensor k, Tensor v, Tensor($1! -> )? out, Tensor cu_seqlens_q, Tensor cu_seqlens_k, Tensor? seqused_k, Tensor? leftpad_k, Tensor? block_table, Tensor? alibi_slopes, int max_seqlen_q, int max_seqlen_k, float p_dropout, float softmax_scale, bool zero_tensors, bool is_causal, int window_size_left, int window_size_right, float softcap, bool return_softmax, Generator? gen) -> Tensor[]

introduced in vllm-project/flash-attention#75

Test Plan

test on A100 before and after PR

Test Result

before vllm crashes on first request; after this PR its fine

(Optional) Documentation Update

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[gemini-code-assist]

Code Review

This pull request resolves a RuntimeError in the FlashAttention 2 C++ extension by updating the vllm-flash-attn dependency to a specific commit that contains the necessary bug fix. The change is correct and well-contained. The approach of updating the GIT_TAG in the CMake build configuration is appropriate for incorporating this fix. The PR appears solid and ready for merging.

[DarkLight1337]

Thanks for the quick fix!