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* Support unpadded LSE layout.
Co-authored-by: Xinfeng Xie <xfxie.ceca@gmail.com>
Co-authored-by: Jianyu Huang <hjyahead@gmail.com>

* Cleanup

* Fix unpadded LSE on split-kv path

* Fix formatting and comments

* Fix inline vs forceinline

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Co-authored-by: Xinfeng Xie <xfxie.ceca@gmail.com>
Co-authored-by: Jianyu Huang <hjyahead@gmail.com>

* Enable paged attention in varlen forward

* Format + fix padding

For faster and smaller builds in some simple cases,
provide switches to allow disabling
-backward
-alibi
-uneven k
-dropout
-local attention
Co-authored-by: Jeremy Francis Reizenstein <bottler@users.noreply.github.com>

Co-authored-by: ljss <450993438@qq.com>

* Add split-k, M<->H to varseq path

* skip M<->H when dropout>0, fix LSE