Update `no_grad` doc examples (#26481)

test=document_fix

python/paddle/fluid/dygraph/base.py

@@ -182,19 +182,19 @@ class no_grad:
      .. code-block:: python
 
         import numpy as np
-        import paddle.fluid as fluid
+        import paddle
 
-        paddle.enable_imperative()
+        paddle.disable_static()
 
         # use as generator
 
         data = np.array([[2, 3], [4, 5]]).astype('float32')
-        l0 = fluid.Linear(2, 2)  # l0.weight.gradient() is None
-        l1 = fluid.Linear(2, 2)
-        with fluid.no_grad():
+        l0 = paddle.nn.Linear(2, 2)  # l0.weight.gradient() is None
+        l1 = paddle.nn.Linear(2, 2)
+        with paddle.no_grad():
             # l1.weight.stop_gradient is False
             tmp = l1.weight * 2  # tmp.stop_gradient is True
-        x = fluid.dygraph.to_variable(data)
+        x = paddle.to_tensor(data)
         y = l0(x) + tmp
         o = l1(y)
         o.backward()
@@ -203,12 +203,12 @@ class no_grad:
 
         # use as decorator
 
-        @fluid.no_grad()
+        @paddle.no_grad()
         def test_layer():
             inp = np.ones([3, 1024], dtype='float32')
-            t = fluid.dygraph.base.to_variable(inp)
-            linear1 = fluid.Linear(1024, 4, bias_attr=False)
-            linear2 = fluid.Linear(4, 4)
+            t = paddle.to_tensor(inp)
+            linear1 = paddle.nn.Linear(1024, 4, bias_attr=False)
+            linear2 = paddle.nn.Linear(4, 4)
             ret = linear1(t)
             dy_ret = linear2(ret)