[Attention] FlashAttention ViT support, make default backend

[MatthewBonanni]

Purpose

This PR is paired with vllm-project/flash-attention#109 (merge that first after CI passes, then I'll update the git tag), which enables FA to support the head sizes required for vision transformers (40, 72, and 80). This PR also updates the selector to make FlashAttention the default backend over xFormers.

Test Plan

pytest tests/kernels/attention/test_flash_attn.py (updated with new head sizes)

Test Result

Passes

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[gemini-code-assist]

Code Review

This pull request updates FlashAttention to support head sizes required for Vision Transformers (40, 72, 80). This is achieved by updating the dependency to a fork of flash-attention, generalizing the head size check in the FlashAttention backend, and updating tests. The logic for selecting the ViT attention backend is also refactored for clarity. My review has identified two main points. First, a critical issue in cmake/external_projects/vllm_flash_attn.cmake where the dependency points to a personal fork, which must be reverted before merging. Second, a high-severity issue in tests/kernels/attention/test_flash_attn.py where a test case for soft_cap has been removed, potentially hiding a feature regression. The other changes look good.

[LucasWilkinson]

Do you know if FA2 is supported too? do you mine testing this on Ampere? I think it should be ok