fix sample code, test=develop (#23448)

python/paddle/fluid/dygraph/layers.py

@@ -122,6 +122,7 @@ class Layer(core.Layer):
             .. code-block:: python
 
               import paddle.fluid as fluid
+              import numpy as np
 
               # the forward_post_hook change the output of the layer: output = output * 2 
               def forward_post_hook(layer, input, output):
@@ -136,15 +137,15 @@ class Layer(core.Layer):
                   # register the hook
                   forward_post_hook_handle = linear.register_forward_post_hook(forward_post_hook)
                   
-                  value = np.arange(26).reshape(2, 13).astype("float32")
+                  value1 = np.arange(26).reshape(2, 13).astype("float32")
-                  in = fluid.dygraph.to_variable(value0)
+                  in1 = fluid.dygraph.to_variable(value1)
                   
-                  out0 = linear(in)
+                  out0 = linear(in1)
                   
                   # remove the hook
                   forward_post_hook_handle.remove()
 
-                  out1 = linear(in)
+                  out1 = linear(in1)
 
                   # hook change the linear's output to output * 2, so out0 is equal to out1 * 2.
                   assert (out0.numpy() == (out1.numpy()) * 2).any()
@@ -173,6 +174,7 @@ class Layer(core.Layer):
             .. code-block:: python
 
               import paddle.fluid as fluid
+              import numpy as np
 
               # the forward_post_hook change the input of the layer: input = input * 2
               def forward_pre_hook(layer, input):