fix bugs

python/paddle/v2/fluid/backward.py

@@ -142,12 +142,13 @@ def _remove_no_grad_branch_(op_descs, no_grad_set):
     """
 
     def _op_can_be_removed_(op_desc, no_grad_set):
-        if _all_in_set_(op_desc.output_arg_names(), no_grad_set):
+        out_arg_names = op_desc.output_arg_names()
+        if len(out_arg_names) == 0 or _all_in_set_(out_arg_names, no_grad_set):
             return True
         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(op_desc.output_arg_names())
+            no_grad_set.union(out_arg_names)
             return True
         return False
 
@@ -296,7 +297,9 @@ def append_backward(loss, parameter_list=None, no_grad_set=None):
                     block_no_grad_set.add(_append_grad_suffix_(var.name))
             no_grad_dict[block.idx] = block_no_grad_set
     elif isinstance(no_grad_set, set):
-        no_grad_dict = {0: no_grad_set}
+        no_grad_dict = {
+            0: set([_append_grad_suffix_(name) for name in no_grad_set])
+        }
     else:
         raise ValueError("'no_grad_set' should be a set or None.")