Update the demo of paddle.grad (#26498)

* update the demo codes of paddle.grad.

python/paddle/fluid/dygraph/base.py

@@ -375,16 +375,16 @@ def grad(outputs,
     Examples 1:
         .. code-block:: python
 
-            import paddle.fluid as fluid
+            import paddle
+            paddle.disable_static()
 
             def test_dygraph_grad(create_graph):
-                with fluid.dygraph.guard(): 
-                    x = fluid.layers.ones(shape=[1], dtype='float32') 
+                x = paddle.ones(shape=[1], dtype='float32')
                 x.stop_gradient = False
                 y = x * x
 
                 # Since y = x * x, dx = 2 * x
-                    dx = fluid.dygraph.grad(
+                dx = paddle.grad(
                         outputs=[y],
                         inputs=[x],
                         create_graph=create_graph,
@@ -410,12 +410,11 @@ def grad(outputs,
     Examples 2:
         .. code-block:: python
 
-            import paddle.fluid as fluid
-
-            fluid.enable_dygraph()
+            import paddle
+            paddle.disable_static()
 
             def test_dygraph_grad(grad_outputs=None):
-                x = fluid.layers.fill_constant(shape=[1], value=2.0, dtype='float32')
+                x = paddle.fill_constant(shape=[1], value=2.0, dtype='float32')
                 x.stop_gradient = False
 
                 y1 = x * x
@@ -431,27 +430,27 @@ def grad(outputs,
                 # Therefore, the final result would be:
                 # dx = 2 * x * dy1 + 3 * dy2 = 4 * dy1 + 3 * dy2.
 
-                dx = fluid.dygraph.grad(
+                dx = paddle.grad(
                     outputs=[y1, y2], 
                     inputs=[x],
                     grad_outputs=grad_outputs)[0]
 
                 return dx.numpy()
 
-            THREE = fluid.layers.fill_constant(shape=[1], value=3.0, dtype='float32')
-            FOUR = fluid.layers.fill_constant(shape=[1], value=4.0, dtype='float32')
+            grad_value = paddle.fill_constant(shape=[1], value=4.0, dtype='float32')
 
             # dy1 = [1], dy2 = [1]
             print(test_dygraph_grad(None)) # [7.]
 
             # dy1 = [1], dy2 = [4]
-            print(test_dygraph_grad([None, FOUR])) # [16.] 
+            print(test_dygraph_grad([None, grad_value])) # [16.]
 
             # dy1 = [4], dy2 = [1]
-            print(test_dygraph_grad([FOUR, None])) # [19.]
+            print(test_dygraph_grad([grad_value, None])) # [19.]
 
             # dy1 = [3], dy2 = [4]
-            print(test_dygraph_grad([THREE, FOUR])) # [24.]
+            grad_y1 = paddle.fill_constant(shape=[1], value=3.0, dtype='float32')
+            print(test_dygraph_grad([grad_y1, grad_value])) # [24.]
 	'''
 
     def check_in_out(in_out_list, name):