[Misc] Add FA2 support to ViT MHA layer

[Isotr0py]

• Add FA2 support to ViT MHA layer.
• I leave Qwen2-VL's ViT MHA layer consolidation to be done in a separate PR, because it has head_size=80 and doesn't work with vllm_flash_attn (head_size should be a multiple of 32).

[github-actions]

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[DarkLight1337]

QQ: Do we have tests for MHA layer?

[Isotr0py]

There is no test for MHA layer yet, we can add one to kernel tests in this PR later.

[DarkLight1337]

LGTM once you get the tests to pass