load_checkpoint support normal dict checkpoints (#351)

* load_checkpoint support normal dict checkpoints

* comments

mmcv/runner/checkpoint.py

@@ -222,14 +222,15 @@ def load_checkpoint(model,
         dict or OrderedDict: The loaded checkpoint.
     """
     checkpoint = _load_checkpoint(filename, map_location)
+    # OrderedDict is a subclass of dict
+    if not isinstance(checkpoint, dict):
+        raise RuntimeError(
+            f'No state_dict found in checkpoint file {filename}')
     # get state_dict from checkpoint
-    if isinstance(checkpoint, OrderedDict):
-        state_dict = checkpoint
-    elif isinstance(checkpoint, dict) and 'state_dict' in checkpoint:
+    if 'state_dict' in checkpoint:
         state_dict = checkpoint['state_dict']
     else:
-        raise RuntimeError(
-            f'No state_dict found in checkpoint file {filename}')
+        state_dict = checkpoint
     # strip prefix of state_dict
     if list(state_dict.keys())[0].startswith('module.'):
         state_dict = {k[7:]: v for k, v in checkpoint['state_dict'].items()}