Fix a backward bug

python/paddle/fluid/backward.py

@@ -136,7 +136,9 @@ def _addup_repetitive_outputs_(op_descs):
                     "sum", {"X": renamed_vars[var_name]}, {"Out": [var_name]},
                     {"use_mkldnn": False}), idx))
                 renamed_vars[var_name] = [var_name]
-        for var_name in op_desc.output_arg_names():
+        for param_name in op_desc.output_names():
+            arg_names = op_desc.output(param_name)
+            for arg_idx, var_name in enumerate(arg_names):
                 if var_name == core.empty_var_name(
                 ) or var_name in op_desc.input_arg_names():
                     # empty variable or inplace op
@@ -157,7 +159,8 @@ def _addup_repetitive_outputs_(op_descs):
                     new_name = var_name + "@RENAME@" + \
                         str(var_rename_count[var_name])
                     var_rename_count[var_name] += 1
-                op_desc.rename_output(var_name, new_name)
+                    arg_names[arg_idx] = new_name
+                    op_desc.set_output(param_name, arg_names)
                     renamed_vars[var_name].append(new_name)
     for var_name, inputs in renamed_vars.iteritems():
         if len(inputs) > 1: