[Optimization] Decode attention support

[lizhenyun01]

Motivation

• attention优化及重构第一部分：
  • attention重构，合并投机解码/非投机解码分支，消除冗余逻辑
  • 拆分decoder_write_cache_with_rope为单独算子，便于维护
  • 新增decode attention backend，当前只支持PD分离下D节点
  • 优化decode attention C8kernel性能，优化后在group_size=14下单步投机场景性能提升5%-113%



• TODO：
  • ROPE，write_cache重构及投机解码等分支融合
  • C16 C4支持
  • 单测完善
  • backend逐步替换append_attention

Checklist

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[paddle-bot]

Thanks for your contribution!