Fix variable cn doc (#1485)

* add astype api in Variable and refine code style

* add astype api in Variable and refine code style

* add astype api in Variable and refine code style

* refine doc

doc/fluid/api_cn/fluid_cn/Variable_cn.rst

@@ -22,8 +22,7 @@ Variable
 
 **示例代码：**
 
-        在静态图形模式下：
-
+在静态图形模式下：
     .. code-block:: python
 
         import paddle.fluid as fluid
@@ -32,8 +31,7 @@ Variable
         new_variable = cur_block.create_var(name="X",
                                             shape=[-1, 23, 48],
                                             dtype='float32')
-        在 `Dygraph <../../user_guides/howto/dygraph/DyGraph.html>`_ 模式下：
-
+在 `Dygraph <../../user_guides/howto/dygraph/DyGraph.html>`_ 模式下：
     .. code-block:: python
 
         import paddle.fluid as fluid
@@ -245,7 +243,7 @@ Variable
 
 **注意：**
 
-  **1. 该API参数只在非** `Dygraph <../../user_guides/howto/dygraph/DyGraph.html>`_ **模式下生效**
+  **1. 该API只在非** `Dygraph <../../user_guides/howto/dygraph/DyGraph.html>`_ **模式下生效**
 
 获取该 :ref:`api_guide_Variable` 的静态描述字符串
 
@@ -279,6 +277,50 @@ Variable
         print(new_variable.to_string(True, True))
 
 
+.. py:method:: astype(self, dtype)
+
+将该 :ref:`api_guide_Variable` 中的数据转换成目标 ``Dtype``
+
+**参数：**
+ - **self** ( :ref:`api_guide_Variable` ) - 当前 :ref:`api_guide_Variable` ， 用户不需要传入。
+ - **dtype** (int | float | float64) - 希望转换成的 ``Dtype``
+
+
+返回：一个全新的转换了 ``Dtype`` 的 :ref:`api_guide_Variable`
+
+返回类型： :ref:`api_guide_Variable`
+
+
+**示例代码**
+
+在静态图模式下：
+    .. code-block:: python
+
+        import paddle.fluid as fluid
+
+        startup_prog = fluid.Program()
+        main_prog = fluid.Program()
+        with fluid.program_guard(startup_prog, main_prog):
+            original_variable = fluid.data(name = "new_variable", shape=[2,2], dtype='float32')
+            new_variable = original_variable.astype('int64')
+            print("new var's dtype is: {}".format(new_variable.dtype))
+
+
+在 `Dygraph <../../user_guides/howto/dygraph/DyGraph.html>`_ 模式下：
+    .. code-block:: python
+
+        import paddle.fluid as fluid
+        import numpy as np
+
+        x = np.ones([2, 2], np.float32)
+        with fluid.dygraph.guard():
+            original_variable = fluid.dygraph.to_variable(x)
+            print("original var's dtype is: {}, numpy dtype is {}".format(original_variable.dtype, original_variable.numpy().dtype))
+            new_variable = original_variable.astype('int64')
+            print("new var's dtype is: {}, numpy dtype is {}".format(new_variable.dtype, new_variable.numpy().dtype))
+
+
+
 属性
 ::::::::::::