!3417 fix bug of group lr when save ckpt

Merge pull request !3417 from wangnan39/fix_bug_of_group_lr_when_save_ckpt

mindspore/nn/optim/optimizer.py

@@ -130,7 +130,7 @@ class Optimizer(Cell):
             if self.dynamic_lr:
                 self.learning_rate = CellList(self.group_lr)
             else:
-                self.learning_rate = tuple(self.group_lr)
+                self.learning_rate = ParameterTuple(self.group_lr)
         else:
             self.learning_rate = self._build_single_lr(learning_rate, 'learning_rate')
         if self.is_group: