fix sample code error of API,test=develop (#2217)

doc/fluid/api_cn/dygraph_cn/NaturalExpDecay_cn.rst

@@ -50,12 +50,14 @@ NaturalExpDecay
     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))
+                    learning_rate=base_lr,
+                    decay_steps=10000,
+                    decay_rate=0.5,
+                    staircase=True),
+                parameter_list=emb.parameters())
 
 
 

doc/fluid/api_cn/dygraph_cn/load_dygraph_cn.rst

@@ -30,9 +30,13 @@ load_dygraph
 
     with fluid.dygraph.guard():
         emb = fluid.dygraph.Embedding([10, 10])
+
         state_dict = emb.state_dict()
         fluid.save_dygraph( state_dict, "paddle_dy")
-        adam = fluid.optimizer.Adam( learning_rate = fluid.layers.noam_decay( 100, 10000) )
+
+        adam = fluid.optimizer.Adam( learning_rate = fluid.layers.noam_decay( 100, 10000),
+                                    parameter_list = emb.parameters() )
+
         state_dict = adam.state_dict()
         fluid.save_dygraph( state_dict, "paddle_dy")