Fix error while params_grads[1]==None

python/paddle/fluid/optimizer.py

@@ -649,19 +649,22 @@ class ModelAverage(Optimizer):
         self.min_average_window = min_average_window
         self.max_average_window = max_average_window
         self.params_grads = params_grads
-        for param, _ in self.params_grads:
+        for param, grad in self.params_grads:
+            if grad is not None:
                 self._append_average_accumulate_op(param)
 
         self.apply_program = Program()
         block = self.apply_program.global_block()
         with program_guard(main_program=self.apply_program):
             for param_grad in self.params_grads:
+                if param_grad[1] is not None:
                     self._add_average_apply_op(block, param_grad)
 
         self.restore_program = Program()
         block = self.restore_program.global_block()
         with program_guard(main_program=self.restore_program):
             for param_grad in self.params_grads:
+                if param_grad[1] is not None:
                     self._add_average_restore_op(block, param_grad)
 
     def _add_average_apply_op(self, block, param_grad):