bug fix for the latest torchvision

mmcv/runner/checkpoint.py

@@ -107,6 +107,7 @@ def load_checkpoint(model,
                 torchvision.models.__path__):
             if not ispkg:
                 _zoo = import_module('torchvision.models.{}'.format(name))
+                if hasattr(_zoo, 'model_urls'):
                     _urls = getattr(_zoo, 'model_urls')
                     model_urls.update(_urls)
         model_name = filename[11:]