[Router] fix interrupt from terminal

[ByronHsu]

The key interrupt from the terminal is not handled properly, causing uncleaned subprocesses. This issue has two main scenarios:

1. Without os.setpgrp():
   All processes share the same Process Group ID (PGID). When you press Ctrl+C, the terminal sends SIGINT to all processes in the group simultaneously. This can cause leaf processes to terminate first, disrupting the proper cleanup order and producing orphaned processes.

Terminal (PGID=100)
└── Main Python Process (PGID=100)
    └── Server Process 1 (PGID=100)
        └── Scheduler 1
        └── Detokenizer 1
    └── Server Process 2 (PGID=100)
        └── Scheduler 2
        └── Detokenizer 2

2. With os.setpgrp():
   The main Python process and its children are placed in separate groups. The process hierarchy becomes:

Terminal (PGID=100)
└── Main Python Process (PGID=200)
    └── Server Process 1 (PGID=300)
        └── Scheduler 1
        └── Detokenizer 1
    └── Server Process 2 (PGID=400)
        └── Scheduler 2
        └── Detokenizer 2

By setting new process group IDs, this issue can be resolved.