[Bugfix] detect alibi and revert to FA2

[tjohnson31415]

On Hopper, Flash Attention 3 is enabled by default (since the v0.7.0 release), but FA3 does not support ALiBi positional encodings. This PR will revert to FA2 if alibi_slopes are passed in to FlashAttentionBackend.

In the v0.8.1 release, the error when attempting to use a model with FA3 with ALiBi (such as bigscience/bloom-1b1) is:

RuntimeError: If cu_seqlens_k is passed in, then page table is not supported

Now there is change in vllm_flash_attn that will raise an AssertionError if alibi is used with FA3. This improves the error message, but the server would still fail to come up. With this PR, vLLM will fall back to FA2 instead of erroring.

FIX #13810

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @tjohnson31415.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[LucasWilkinson]

LGTM, good catch, thanks for the contribution!