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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.