[Bugfix] Remove xformers requirement for Pixtral

[mgoin]

This PR adds a torch SDPA implementation for PixtralHF attention

FIX #9036 (comment)

I'm trying to run LlavaNextForConditionalGeneration on a non-CUDA hardware platform, but when I import llava_next, it pulls in pixtral, which in turn imports xformers. From my understanding, xformers is required only for CUDA support. Is there a way to avoid this dependency or run LlavaNextForConditionalGeneration without xformers on non-CUDA platforms?

[github-actions]

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