check port availability only in main deepspeed/torchrun launcher (#2078)

* check port availability only in main deepspeed launcher

* check port availability only in main launcher for deepspeed/torchrun

* Update launch.py

add comments

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Co-authored-by: 聂靖入 <niejingru@bytedance.com>

src/accelerate/utils/launch.py

@@ -128,7 +128,10 @@ def prepare_multi_gpu_env(args: argparse.Namespace) -> Dict[str, str]:
     if main_process_port is None:
         main_process_port = 29500
 
-    if is_port_in_use(main_process_port):
+    # only need to check port availability in main process, in case we have to start multiple launchers on the same machine
+    # for some reasons like splitting log files.
+    need_port_check = num_machines <= 1 or int(args.machine_rank) == 0
+    if need_port_check and is_port_in_use(main_process_port):
         raise ConnectionError(
             f"Tried to launch distributed communication on port `{main_process_port}`, but another process is utilizing it. "
             "Please specify a different port (such as using the `----main_process_port` flag or specifying a different `main_process_port` in your config file)"
@@ -272,7 +275,10 @@ def prepare_deepspeed_cmd_env(args: argparse.Namespace) -> Tuple[List[str], Dict
     if main_process_port is None:
         main_process_port = 29500
 
-    if is_port_in_use(main_process_port):
+    # only need to check port availability in main process, in case we have to start multiple launchers on the same machine
+    # for some reasons like splitting log files.
+    need_port_check = num_machines <= 1 or int(args.machine_rank) == 0
+    if need_port_check and is_port_in_use(main_process_port):
         raise ConnectionError(
             f"Tried to launch distributed communication on port `{main_process_port}`, but another process is utilizing it. "
             "Please specify a different port (such as using the `----main_process_port` flag or specifying a different `main_process_port` in your config file)"