Modify doc mistakes of grad API. (#29176)

python/paddle/fluid/dygraph/base.py

@@ -444,7 +444,6 @@ def grad(outputs,
         .. code-block:: python
 
             import paddle
-            paddle.disable_static()
 
             def test_dygraph_grad(create_graph):
                 x = paddle.ones(shape=[1], dtype='float32')
@@ -479,10 +478,9 @@ def grad(outputs,
         .. code-block:: python
 
             import paddle
-            paddle.disable_static()
 
             def test_dygraph_grad(grad_outputs=None):
-                x = paddle.fluid.layers.fill_constant(shape=[1], value=2.0, dtype='float32')
+                x = paddle.to_tensor(2.0)
                 x.stop_gradient = False
 
                 y1 = x * x
@@ -505,8 +503,7 @@ def grad(outputs,
 
                 return dx.numpy()
 
-            grad_value = paddle.fluid.layers.fill_constant(shape=[1], value=4.0, dtype='float32')
-
+            grad_value = paddle.to_tensor(4.0)
             # dy1 = [1], dy2 = [1]
             print(test_dygraph_grad(None)) # [7.]
 
@@ -517,7 +514,7 @@ def grad(outputs,
             print(test_dygraph_grad([grad_value, None])) # [19.]
 
             # dy1 = [3], dy2 = [4]
-            grad_y1 = paddle.fluid.layers.fill_constant(shape=[1], value=3.0, dtype='float32')
+            grad_y1 = paddle.to_tensor(3.0)
             print(test_dygraph_grad([grad_y1, grad_value])) # [24.]
 	'''