[BugFix] Support initialize llumlet by manager

[s5u13b]

When the migration backend is gloo, during scale_up/scale_down, llumnix needs to re-initialize the migration communication process group. However, Ray requires that all the processes that attempt to initialize the process group share the same driver process to ensure that the job_id is consistent. This means that all the llumlets should be initialized in the same process in llumnix. Previously, llumnix initialize llumlet in the entrypoint process, and we can have multiple entrypoints in llumnix, so the driver process of llumlets can be different. Therefore, we support initializing llumlet by the manager as another option in llumnix to ensure that all the llumlets can share the same driver process(manager).