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* Softcap v2 (fwd only).

* Some missing interface + remove overrides in tests.

Co-authored-by: moshuosha <moshuosha@qq.com>

* 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

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

Co-authored-by: Timothee Lacroix <t@mistral.ai>

Since we could be parallelizing over seqlen_k

This is faster since we only need to do atomic adds on dq, instead of atomic
adds on both dk and dv.

WIP because there seems to be some race conditions for head dimensions other
than 16, 32, 64, 128.