[Cherry-Pick][CI][BugFix]Fix attention mask bug in D-Node of PD-split mode(#5245)

[freeliuzc]

Motivation

1. 在PD分离下，attn_mask_offset 会置为错误的值(比实际值大1)
2. MTP 与 Target model 是类似的情况

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💡 如若此PR是Cherry Pick，PR标题需遵循格式，在最开始加上[Cherry-Pick]标签，以及最后面加上原PR ID，例如[Cherry-Pick][CI] Add check trigger and logic(#5191)

Modifications

1. 当 Query 在 D节点第一次插入时，通过 rollback 回溯一步

#5245

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]]
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• 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.
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[paddle-bot]

Thanks for your contribution!

[carryyu]

LGTM

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/online/20251131@49be443). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/spec_decode/mtp.py	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##             release/online/20251131    #5246   +/-   ##
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  Coverage                           ?   59.11%           
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  Files                              ?      317           
  Lines                              ?    38690           
  Branches                           ?     5816           
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  Hits                               ?    22872           
  Misses                             ?    13992           
  Partials                           ?     1826           

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