Update `no_grad` examples (#2426)

doc/fluid/api_cn/dygraph_cn/no_grad_cn.rst

@@ -21,19 +21,21 @@ no_grad
 ..  code-block:: python
 
     import numpy as np
-    import paddle.fluid as fluid
+    import paddle
+
+    paddle.disable_static()
 
     paddle.enable_imperative()
 
     # 用作生成器
 
     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()
@@ -41,13 +43,12 @@ no_grad
     print(l0.weight.gradient() is None)  # False
 
     # 用作装饰器
-
-    @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)