Update `no_grad` docs (#2324)

* Update `no_grad` docs

add alias `paddle.no_grad`
use `paddle.no_grad()` for decorator usage
update code sample

* Fix alias

* Reword decorator usage note

doc/fluid/api_cn/dygraph_cn/no_grad_cn.rst

@@ -4,48 +4,51 @@ no_grad
 -------------------------------
 
 
-.. py:method:: paddle.fluid.dygraph.no_grad(func=None)
+.. py:class:: paddle.fluid.dygraph.no_grad
 
 :api_attr: 命令式编程模式（动态图)
-
+:alias_main: paddle.no_grad
+:alias: paddle.no_grad
+:old_api: paddle.fluid.dygraph.no_grad
 
 
 创建一个上下文来禁用动态图梯度计算。在此模式下，每次计算的结果都将具有stop_gradient=True。
 
-也可以用作一个装饰器（确保不要用括号来初始化）。
+也可以用作一个装饰器（需要创建实例对象作为装饰器）。
 
 **代码示例**
 
 ..  code-block:: python
 
-
     import numpy as np
     import paddle.fluid as fluid
 
+    paddle.enable_imperative()
+
     # 用作生成器
+
     data = np.array([[2, 3], [4, 5]]).astype('float32')
-    with fluid.dygraph.guard():
-        l0 = fluid.Linear(2, 2)  # l0.weight.gradient() is None
-        l1 = fluid.Linear(2, 2)
-        with fluid.dygraph.no_grad():
-            # l1.weight.stop_gradient is False
-            tmp = l1.weight * 2  # tmp.stop_gradient is True
-        x = fluid.dygraph.to_variable(data)
-        y = l0(x) + tmp
-        o = l1(y)
-        o.backward()
-        print(tmp.gradient() is None)  # True
-        print(l0.weight.gradient() is None)  # False
-    
+    l0 = fluid.Linear(2, 2)  # l0.weight.gradient() is None
+    l1 = fluid.Linear(2, 2)
+    with fluid.no_grad():
+        # l1.weight.stop_gradient is False
+        tmp = l1.weight * 2  # tmp.stop_gradient is True
+    x = fluid.dygraph.to_variable(data)
+    y = l0(x) + tmp
+    o = l1(y)
+    o.backward()
+    print(tmp.gradient() is None)  # True
+    print(l0.weight.gradient() is None)  # False
+
     # 用作装饰器
-    @fluid.dygraph.no_grad
+
+    @fluid.no_grad()
     def test_layer():
-        with fluid.dygraph.guard():
-            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)
-            ret = linear1(t)
-            dy_ret = linear2(ret)
+        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)
+        ret = linear1(t)
+        dy_ret = linear2(ret)
 
     test_layer()

doc/fluid/api_cn/paddle_cn.rst

@@ -5,7 +5,7 @@ paddle
 ..  toctree::
 	:maxdepth: 1
 
-	paddle_cn/abs_cn.rst
+    paddle_cn/abs_cn.rst
 	paddle_cn/acos_cn.rst
 	paddle_cn/addcmul_cn.rst
 	paddle_cn/addmm_cn.rst
@@ -101,7 +101,8 @@ paddle
         paddle_cn/minimum_cn.rst
 	paddle_cn/multiplex_cn.rst
 	paddle_cn/mul_cn.rst
-	paddle_cn/name_scope_cn.rst
+    paddle_cn/name_scope_cn.rst
+    paddle_cn/no_grad_cn.rst
 	paddle_cn/nonzero_cn.rst
 	paddle_cn/not_equal_cn.rst
 	paddle_cn/ones_cn.rst

doc/fluid/api_cn/paddle_cn/no_grad_cn.rst

+.. _cn_api_paddle_cn_name_scope:
+
+name_scope
+-------------------------------
+:doc_source: paddle.fluid.dygraph.no_grad