Add json set_default for unserializable values (#485)

* Add json set_default for unserializable values

* kwargs.setdefault

mmcv/fileio/handlers/json_handler.py

 # Copyright (c) Open-MMLab. All rights reserved.
 import json
 
+import numpy as np
+
 from .base import BaseFileHandler
 
 
+def set_default(obj):
+    """Set default json values for non-serializable values.
+
+    It helps convert ``set``, ``range`` and ``np.ndarray`` data types to list.
+    """
+    if isinstance(obj, (set, range)):
+        return list(obj)
+    elif isinstance(obj, np.ndarray):
+        return obj.tolist()
+    raise TypeError
+
+
 class JsonHandler(BaseFileHandler):
 
     def load_from_fileobj(self, file):
         return json.load(file)
 
     def dump_to_fileobj(self, obj, file, **kwargs):
+        kwargs.setdefault('default', set_default)
         json.dump(obj, file, **kwargs)
 
     def dump_to_str(self, obj, **kwargs):
+        kwargs.setdefault('default', set_default)
         return json.dumps(obj, **kwargs)