Add ability to overwite existing module in Registry (#1982)

Changes to be committed:
modified:   mmdet/utils/registry.py

mmdet/utils/registry.py

 import inspect
+from functools import partial
 
 import mmcv
 
@@ -25,7 +26,7 @@ class Registry(object):
     def get(self, key):
         return self._module_dict.get(key, None)
 
-    def _register_module(self, module_class):
+    def _register_module(self, module_class, force=False):
         """Register a module.
 
         Args:
@@ -35,13 +36,15 @@ class Registry(object):
             raise TypeError('module must be a class, but got {}'.format(
                 type(module_class)))
         module_name = module_class.__name__
-        if module_name in self._module_dict:
+        if not force and module_name in self._module_dict:
             raise KeyError('{} is already registered in {}'.format(
                 module_name, self.name))
         self._module_dict[module_name] = module_class
 
-    def register_module(self, cls):
-        self._register_module(cls)
+    def register_module(self, cls=None, force=False):
+        if cls is None:
+            return partial(self.register_module, force=force)
+        self._register_module(cls, force=force)
         return cls