[Optimize][Cherry-pick] optimize ttft for EP(#6098)

[rainyfly]

Motivation

优化在 EP 并行下的首 token 问题：

1. 在 forward 结束后调度发下一班车。
2. 发车时没有请求使用空请求同步。

Modifications

None

Usage or Command

None

Accuracy Tests

None

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 52.38095% with 20 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/online/20251131@e29272c). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/worker/worker_process.py	53.84%	5 Missing and 7 partials ⚠️
fastdeploy/engine/common_engine.py	22.22%	7 Missing ⚠️
fastdeploy/scheduler/dp_scheduler.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##             release/online/20251131    #6085   +/-   ##
==========================================================
  Coverage                           ?   57.36%           
==========================================================
  Files                              ?      324           
  Lines                              ?    39388           
  Branches                           ?     5943           
==========================================================
  Hits                               ?    22594           
  Misses                             ?    14968           
  Partials                           ?     1826           

Flag	Coverage Δ
GPU	57.36% <52.38%> (?)

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