fix a bug

python/paddle/v2/fluid/backward.py

@@ -178,7 +178,7 @@ def _remove_no_grad_branch_(op_descs, no_grad_set):
         if _all_in_set_(
                 filter(lambda name: name.find(core.grad_var_suffix()) != -1,
                        op_desc.input_arg_names()), no_grad_set):
-            no_grad_set.union(out_arg_names)
+            no_grad_set.update(out_arg_names)
             return True
         return False