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* start varlen variants for attn backend kernels.

* maybe unflatten heads.

* updates

* remove unused function.

* doc

* up

* up

* support automatic dispatch.

* disable compile support for now./

* up

* flash too.

* document.

* up

* up

* up

* up

* add aiter attention backend

* Apply style fixes

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Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

* checks

* feedback

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Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

* slight edits to the attention backends docs.

* Update docs/source/en/optimization/attention_backends.md
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

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Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* init

* feedback

* update

* feedback

* fixes