[BugFix] Fix step_shm_value in PD disaggregated deployment

[liyonghua0910]

Motivation

修复 #4693 遗留的问题：为了修复 v1 调度下 PD 分离的启动问题， 将 step_shm_value 修改成了通过 shared_memory_exists() 判断是否要临时创建该信号量。但实测这种写法会在压测途中挂掉。

Modifications

Worker 在 v0 下不需要创建该信号量，在 v1 下不应该去读取该信号量：

if not envs.ENABLE_V1_KVCACHE_SCHEDULER and self.scheduler_config.splitwise_role == "prefill":
    step_shm_value = IPCSignal(..., create=False)

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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• 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!

[K11OntheBoat]

LGTM