fix sample code error of fluid.dygraph.NaturalExpDecay,test=develop (#25092)

fix sample code error of fluid.dygraph.NaturalExpDecay

python/paddle/fluid/dygraph/learning_rate_scheduler.py

@@ -178,12 +178,14 @@ class NaturalExpDecay(LearningRateDecay):
             import paddle.fluid as fluid
             base_lr = 0.1
             with fluid.dygraph.guard():
+                emb = fluid.dygraph.Embedding([10, 10])
                 sgd_optimizer = fluid.optimizer.SGD(
                         learning_rate=fluid.dygraph.NaturalExpDecay(
                             learning_rate=base_lr,
                             decay_steps=10000,
                             decay_rate=0.5,
-		            staircase=True))
+                            staircase=True),
+                        parameter_list=emb.parameters())
 
     """