add len to DC (#320)

Co-authored-by: lixuanyi <lixuanyi@sensetime.com>

mmcv/parallel/data_container.py

@@ -50,6 +50,9 @@ class DataContainer(object):
     def __repr__(self):
         return f'{self.__class__.__name__}({repr(self.data)})'
 
+    def __len__(self):
+        return len(self._data)
+
     @property
     def data(self):
         return self._data