[Misc] Black auto fix. (#4680)

* [Misc] Black auto fix.

* fix pylint disable
Co-authored-by: Steve <ubuntu@ip-172-31-34-29.ap-northeast-1.compute.internal>

python/dgl/backend/set_default_backend.py

 import argparse
-import os
 import json
+import os
+
 
 def set_default_backend(default_dir, backend_name):
     os.makedirs(default_dir, exist_ok=True)
-    config_path = os.path.join(default_dir, 'config.json')
+    config_path = os.path.join(default_dir, "config.json")
     with open(config_path, "w") as config_file:
-        json.dump({'backend': backend_name.lower()}, config_file)
-    print('Setting the default backend to "{}". You can change it in the '
-          '~/.dgl/config.json file or export the DGLBACKEND environment variable.  '
-          'Valid options are: pytorch, mxnet, tensorflow (all lowercase)'.format(
-              backend_name))
+        json.dump({"backend": backend_name.lower()}, config_file)
+    print(
+        'Setting the default backend to "{}". You can change it in the '
+        "~/.dgl/config.json file or export the DGLBACKEND environment variable.  "
+        "Valid options are: pytorch, mxnet, tensorflow (all lowercase)".format(
+            backend_name
+        )
+    )
+
 
 if __name__ == "__main__":
     parser = argparse.ArgumentParser()
-    parser.add_argument("default_dir", type=str, default=os.path.join(os.path.expanduser('~'), '.dgl'))
-    parser.add_argument("backend", nargs=1, type=str, choices=[
-                        'pytorch', 'tensorflow', 'mxnet'], help="Set default backend")
+    parser.add_argument(
+        "default_dir",
+        type=str,
+        default=os.path.join(os.path.expanduser("~"), ".dgl"),
+    )
+    parser.add_argument(
+        "backend",
+        nargs=1,
+        type=str,
+        choices=["pytorch", "tensorflow", "mxnet"],
+        help="Set default backend",
+    )
     args = parser.parse_args()
     set_default_backend(args.default_dir, args.backend[0])

python/dgl/backend/tensorflow/__init__.py

 import os
-os.environ['TF_FORCE_GPU_ALLOW_GROWTH'] = 'true'
 
-from .tensor import *
+os.environ["TF_FORCE_GPU_ALLOW_GROWTH"] = "true"
+
 from .sparse import *
+from .tensor import *