[API 2.0] Fix doc and example code of api 'switch_case' and add/delete alias (#2689)

* Fix example code of api `fluid.layers.switch_case` to use api2.0

* add alias `paddle.static.nn.switch_case`
    
* delete alias `paddle.nn.switch_case` and `paddle.nn.control_flow.switch_case`

doc/paddle/api/alias_api_mapping

@@ -225,7 +225,7 @@ paddle.fluid.layers.pool2d	paddle.nn.functional.pool2d,paddle.nn.functional.pool
 paddle.nn.functional.norm.batch_norm	paddle.nn.functional.batch_norm
 paddle.nn.layer.common.UpsamplingNearest2d	paddle.nn.UpsamplingNearest2d,paddle.nn.layer.UpsamplingNearest2d
 paddle.nn.functional.conv.conv_transpose3d	paddle.nn.functional.conv_transpose3d
-paddle.fluid.layers.switch_case	paddle.nn.switch_case,paddle.nn.control_flow.switch_case
+paddle.fluid.layers.switch_case	paddle.static.nn.switch_case
 paddle.fluid.layers.image_resize	paddle.nn.functional.image_resize,paddle.nn.functional.vision.image_resize
 paddle.nn.layer.conv.Conv3d	paddle.nn.Conv3d,paddle.nn.layer.Conv3d
 paddle.fluid.layers.logical_or	paddle.logical_or,paddle.tensor.logical_or,paddle.tensor.logic.logical_or

doc/paddle/api/paddle/fluid/layers/switch_case_cn.rst

@@ -6,13 +6,13 @@ switch_case
 
 .. py:function:: paddle.fluid.layers.switch_case(branch_index, branch_fns, default=None, name=None)
 
-
+:api_attr: 声明式编程模式（静态图)
 
 
 该OP的运行方式类似于c++的switch/case。
 
 参数：
-    - **branch_index** (Variable)- 形状为[1]的Tensor，指定将要执行的分支。数据类型是 ``int32``, ``int64`` 或 ``uint8``。
+    - **branch_index** (Tensor)- 形状为[1]的Tensor，指定将要执行的分支。数据类型是 ``int32``, ``int64`` 或 ``uint8``。
     - **branch_fns** (dict|list|tuple) - 如果 ``branch_fns`` 是一个list或tuple，它的元素可以是 (int, callable) 二元组，即由整数和可调用对象构成的二元组，整数表示对应的可调用对象的键；也可以仅仅是可调用对象，它在list或者tuple中的实际索引值将作为该可调用对象的键。如果 ``branch_fns`` 是一个字典，那么它的键是整数，它的值是可调用对象。所有的可调用对象都返回相同结构的Tensor。
     - **default** (callable，可选) - 可调用对象，返回一个或多个张量。
     - **name** (str，可选) – 具体用法请参见 :ref:`api_guide_Name` ，一般无需设置，默认值：None。
@@ -20,7 +20,7 @@ switch_case
 返回：如果 ``branch_fns`` 中存在与 ``branch_index`` 匹配的可调用对象，则返回该可调用对象的返回结果；如果 ``branch_fns`` 中不存在与 ``branch_index`` 匹配的可调用对象且 ``default`` 不是None，则返回调用 ``default`` 的返回结果；
 如果 ``branch_fns`` 中不存在与 ``branch_index`` 匹配的可调用对象且 ``default`` 是None，则返回 ``branch_fns`` 中键值最大的可调用对象的返回结果。
 
-返回类型：Variable|list(Variable)
+返回类型：Tensor|list(Tensor)
 
 抛出异常：
     - ``TypeError`` - 如果 ``branch_index`` 的类型不是list或tuple。
@@ -37,43 +37,42 @@ switch_case
 
 .. code-block:: python
 
-    import paddle.fluid as fluid
-    import paddle.fluid.layers as layers
+    import paddle
+
+    paddle.enable_static()
 
     def fn_1():
-        return layers.fill_constant(shape=[1, 2], dtype='float32', value=1)
+        return paddle.fill_constant(shape=[1, 2], dtype='float32', value=1)
 
     def fn_2():
-        return layers.fill_constant(shape=[2, 2], dtype='int32', value=2)
+        return paddle.fill_constant(shape=[2, 2], dtype='int32', value=2)
 
     def fn_3():
-        return layers.fill_constant(shape=[3], dtype='int32', value=3)
+        return paddle.fill_constant(shape=[3], dtype='int32', value=3)
 
-    main_program = fluid.default_startup_program()
-    startup_program = fluid.default_main_program()
-    with fluid.program_guard(main_program, startup_program):
-        index_1 = layers.fill_constant(shape=[1], dtype='int32', value=1)
-        index_2 = layers.fill_constant(shape=[1], dtype='int32', value=2)
+    main_program = paddle.static.default_startup_program()
+    startup_program = paddle.static.default_main_program()
+    with paddle.static.program_guard(main_program, startup_program):
+        index_1 = paddle.fill_constant(shape=[1], dtype='int32', value=1)
+        index_2 = paddle.fill_constant(shape=[1], dtype='int32', value=2)
 
-        out_1 = layers.switch_case(
+        out_1 = paddle.static.nn.switch_case(
             branch_index=index_1,
             branch_fns={1: fn_1, 2: fn_2},
             default=fn_3)
 
-        out_2 = layers.switch_case(
+        out_2 = paddle.static.nn.switch_case(
             branch_index=index_2,
             branch_fns=[(1, fn_1), (2, fn_2)],
             default=fn_3)
 
         # Argument default is None and no index matches. fn_3 will be called because of the max index 7.
-        out_3 = layers.switch_case(
+        out_3 = paddle.static.nn.switch_case(
             branch_index=index_2,
             branch_fns=[(0, fn_1), (4, fn_2), (7, fn_3)])
 
-        exe = fluid.Executor(fluid.CPUPlace())
+        exe = paddle.static.Executor(paddle.CPUPlace())
         res_1, res_2, res_3 = exe.run(main_program, fetch_list=[out_1, out_2, out_3])
         print(res_1)  # [[1. 1.]]
         print(res_2)  # [[2 2] [2 2]]
         print(res_3)  # [3 3 3]
-
-