[Cherry-Pick] [BugFix] fix mtp split kv attetion

[lizhenyun01]

Motivation

修复mtp下 split kv attention 的merge操作未跳过prefill槽位的bug

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

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release/2.4@f3ebd64). Learn more about missing BASE report.

Additional details and impacted files

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##             release/2.4    #5921   +/-   ##
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  Files                  ?      329           
  Lines                  ?    40818           
  Branches               ?     6221           
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  Hits                   ?    24008           
  Misses                 ?    14933           
  Partials               ?     1877           

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