Update annotations of layers.py

python/paddle/trainer_config_helpers/layers.py

@@ -3668,6 +3668,7 @@ def gru_step_naive_layer(input,
     :param param_attr:
     :param layer_attr:
     :return:
+    :rtype: LayerOutput
     """
     if input.size % 3 != 0:
         raise ValueError("GruStep input size must be divided by 3")