Fa upstream3 (#561)

[fsx950223]

• integrate aiter

• add env variable

• rename function

• optimize kernels with small query lens

• change condition

• add rocm aiter backend

• new fa impl

• update api

• optimize performance

• remove try catch

• clean code

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Please direct your PRs to the upstream vllm (https://github.com/vllm-project/vllm.git)

Accepting PRs into the ROCm fork (https://github.com/ROCm/vllm) will require a clear previously communicated exception

[github-actions]

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