[BugFix] Fix for IMA in FA3 varlen combine

[LucasWilkinson]

Purpose

vLLM side of vllm-project/flash-attention#80

Test Plan + Result

@sarckk confirms this fixes his issue; may help with some of the H100 related issues in #19483 (not the A100 related ones; this issue appears to have multiple independent IMAs being discussed)

(Optional) Documentation Update

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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.

[github-actions]

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[gemini-code-assist]

Code Review

This pull request updates the flash-attention dependency to a new commit, presumably to incorporate a bug fix for IMA in FlashAttention v3 as indicated by the title. My review focuses on improving the maintainability of this dependency management. I've suggested adding a comment to the cmake file to provide context for the specific commit hash being used. This will help future developers understand the reason for this pin. It is also highly recommended to provide details on how this change was tested in the pull request description to ensure it fixes the intended bug without introducing regressions, especially given the 'footgun' warning in the cmake file regarding this dependency.

[gemini-code-assist]

For better maintainability, it's good practice to add a comment explaining why this specific commit hash is being used. This provides context for future developers who might need to update this dependency. You could reference the bug fix or the associated pull request.¹

          # From https://github.com/vllm-project/flash-attention/pull/80 for IMA fix in FA3
          GIT_TAG 4c6a539a4d69241c612fea88b6af6c46d05eb542

Style Guide References

Footnotes

1. Code should be well-commented to explain the purpose and origin of 'magic' values like raw commit hashes, which improves long-term maintainability. ↩

[mergify]

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

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

[cjackal]

The fastcheck fails due to temporary aws s3 connection error, FYI.