[Cherry-Pick][Optimization]Decode append attention support(#5767)

[lizhenyun01]

Motivation

• attention优化及重构V1 batch，当前只支持D节点，
• 使用方式：export FD_ATTENTION_BACKEND=DECODE_APPEND_ATTN

Modifications

Usage or Command

Accuracy Tests

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.
• Format your code, run pre-commit before commit.
• 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 (release/online/20251131@3a94e6f). 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                     @@
##             release/online/20251131    #5954   +/-   ##
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  Coverage                           ?   58.40%           
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  Files                              ?      324           
  Lines                              ?    39340           
  Branches                           ?     5935           
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  Hits                               ?    22977           
  Misses                             ?    14515           
  Partials                           ?     1848           

Flag	Coverage Δ
GPU	58.40% <35.21%> (?)

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