Add Sageattention backend

[flozi00]

#9654

[github-actions]

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[flozi00]

@simon-mo @mgoin
What do you think about the deps ?
Copying the kernels to vLLM or using the pip / repo install ?

[simon-mo]

Generally if the attention backend has pip installable package we prefer that. But before investigate further, I'm curious whether there's performance benefit for LLM? Can you run some benchmarks (e.g. benchmark throughput for Llama 8B)? Thank you!