[GenAI] Add FlashAttention

[LittleLittleCloud]

We are excited to review your PR.

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• [x] There's a descriptive title that will make sense to other developers some time from now.
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• [x] Your change description explains what the change does, why you chose your approach, and anything else that reviewers should know.
• [ ] You have included any necessary tests in the same PR.

7169

[dotnet-policy-service[bot]]

Draft Pull Request was automatically closed for 30 days of inactivity. Please let us know if you'd like to reopen it.