keep forward compatibility

Author: luotao1

python/paddle/trainer_config_helpers/layers.py

@@ -53,6 +53,7 @@
     "hsigmoid",
     "conv_projection",
     "mse_cost",
+    "regression_cost",
     'classification_cost',
     "LayerOutput",
     'img_conv_layer',
@@ -3605,6 +3606,9 @@ def mse_cost(input, label, weight=None, name=None, layer_attr=None):
     return LayerOutput(name, LayerType.COST, parents=parents, size=1)
 
 
+regression_cost = mse_cost
+
+
 @wrap_name_default("cost")
 @layer_support()
 def classification_cost(input,