Catch symlink failure on Windows (#6482)

* Catch symlink failure on Windows

Signed-off-by: del-zhenwu <dele.zhenwu@gmail.com>

* Set copy mode on Windows

Signed-off-by: del-zhenwu <dele.zhenwu@gmail.com>

* Fix lint

Signed-off-by: del-zhenwu <dele.zhenwu@gmail.com>

* Fix logic error

Signed-off-by: del-zhenwu <dele.zhenwu@gmail.com>

setup.py

@@ -2,6 +2,7 @@
 # Copyright (c) OpenMMLab. All rights reserved.
 import os
 import os.path as osp
+import platform
 import shutil
 import sys
 import warnings
@@ -141,7 +142,11 @@ def add_mim_extension():
     # parse installment mode
     if 'develop' in sys.argv:
         # installed by `pip install -e .`
-        mode = 'symlink'
+        if platform.system() == 'Windows':
+            # set `copy` mode here since symlink fails on Windows.
+            mode = 'copy'
+        else:
+            mode = 'symlink'
     elif 'sdist' in sys.argv or 'bdist_wheel' in sys.argv:
         # installed by `pip install .`
         # or create source distribution by `python setup.py sdist`