fix multi card

Author: jacquesqiao

python/paddle/fluid/backward.py

@@ -220,7 +220,10 @@ def _callback_lookup_(op):
     :return: callback function
     """
     if op.type == 'parallel_do' and op.attr('use_nccl'):
+        all_vars = op.block.vars
         param_names = set(op.input('parameters'))
+        param_names = filter(lambda name: all_vars[name].stop_gradient is False,
+                             param_names)
         param_grad_names = [n + "@GRAD" for n in param_names]
 
         class ParallelDoCallBack(object):

python/paddle/fluid/layers/control_flow.py

@@ -294,8 +294,7 @@ def get_parameters(self):
 
         params = list(set(params))
 
-        param_list = [parent_block.var(name) for name in params]
-        return filter(lambda param: param.stop_gradient is False, param_list)
+        return [parent_block.var(name) for name in params]
 
     def complete_op(self):
         main_program = self.helper.main_program