[Feature] Support finding free port in _init_dist_slurm() (#1846)

* [feat]:support find free port in _init_dist_slurm

* fix format

* Update mmcv/runner/dist_utils.py

should support port taken by a non-localhost address.
Co-authored-by: Mashiro <57566630+HAOCHENYE@users.noreply.github.com>

* Update dist_utils.py

Add Copyright.

* rename inner function

* Update mmcv/runner/dist_utils.py
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* fix dist_utils.py

change _is_port_in_use() criterion.

* Update dist_utils.py

rename _is_port_in_use to _is_free_port

* Update mmcv/runner/dist_utils.py
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* Update dist_utils.py

fix lint

* Update dist_utils.py

fix lint
Co-authored-by: Mashiro <57566630+HAOCHENYE@users.noreply.github.com>
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

mmcv/runner/dist_utils.py

 # Copyright (c) OpenMMLab. All rights reserved.
+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
 import functools
 import os
+import socket
 import subprocess
 from collections import OrderedDict
 
@@ -11,6 +13,24 @@ from torch._utils import (_flatten_dense_tensors, _take_tensors,
                           _unflatten_dense_tensors)
 
 
+def _find_free_port():
+    # Copied from https://github.com/facebookresearch/detectron2/blob/main/detectron2/engine/launch.py # noqa: E501
+    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    # Binding to port 0 will cause the OS to find an available port for us
+    sock.bind(('', 0))
+    port = sock.getsockname()[1]
+    sock.close()
+    # NOTE: there is still a chance the port could be taken by other processes.
+    return port
+
+
+def _is_free_port(port):
+    ips = socket.gethostbyname_ex(socket.gethostname())[-1]
+    ips.append('localhost')
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+        return all(s.connect_ex((ip, port)) != 0 for ip in ips)
+
+
 def init_dist(launcher, backend='nccl', **kwargs):
     if mp.get_start_method(allow_none=True) is None:
         mp.set_start_method('spawn')
@@ -64,8 +84,12 @@ def _init_dist_slurm(backend, port=None):
     elif 'MASTER_PORT' in os.environ:
         pass  # use MASTER_PORT in the environment variable
     else:
-        # 29500 is torch.distributed default port
+        # if torch.distributed default port(29500) is available
+        # then use it, else find a free port
+        if _is_free_port(29500):
             os.environ['MASTER_PORT'] = '29500'
+        else:
+            os.environ['MASTER_PORT'] = str(_find_free_port())
     # use MASTER_ADDR in the environment variable if it already exists
     if 'MASTER_ADDR' not in os.environ:
         os.environ['MASTER_ADDR'] = addr