[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

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
  • You can add new tags based on the PR content, but the semantics must be clear.
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• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[paddle-bot]

Thanks for your contribution!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 35.21127% with 92 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@fb37423). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ayers/attention/decode_append_attention_backend.py	21.90%	81 Missing and 1 partial ⚠️
fastdeploy/platforms/cuda.py	0.00%	2 Missing and 1 partial ⚠️
fastdeploy/spec_decode/mtp.py	0.00%	1 Missing and 1 partial ⚠️
fastdeploy/worker/gpu_model_runner.py	0.00%	1 Missing and 1 partial ⚠️
...cutor/layers/attention/ops/config_for_attention.py	85.71%	0 Missing and 1 partial ⚠️
...or/layers/attention/ops/decode_append_attention.py	88.88%	0 Missing and 1 partial ⚠️
...ers/attention/ops/decoder_write_cache_with_rope.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #5767   +/-   ##
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  Coverage           ?   66.55%           
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  Files              ?      389           
  Lines              ?    51364           
  Branches           ?     8005           
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  Hits               ?    34186           
  Misses             ?    14720           
  Partials           ?     2458           

Flag	Coverage Δ
GPU	66.55% <35.21%> (?)

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