[BugFix] fix mtp cache attaching for pd disaggregation

[liyonghua0910]

Motivation

修复 PD 分离时 MTP 启动失败问题，在 PD 分离场景需要由 cache messager 创建 cache，而 gpu model runner 需要等待 cache 创建完成的信号，才能开始读取 cache。

此前的 PR (#5840) 把 mtp 模块的 cache 初始化提前到了主模型 cache 初始化前面，主模型有这个等待逻辑，而 mtp 模型中没有，导致初始化出错。

Modifications

将 cache 的等待逻辑从 gpu_model_runner.py 同步到 mtp.py 文件

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 46.15385% with 7 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@e911ac2). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/spec_decode/mtp.py	46.15%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
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  Hits               ?    29630           
  Misses             ?    12607           
  Partials           ?     2184           

Flag	Coverage Δ
GPU	66.70% <46.15%> (?)

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