[API 2.0] Fix example code of api 'case' and add/delete alias (#27577)

* Fix example code of api fluid.layers.case

* delete paddle.nn.case alias and add paddle.static.nn.case

python/paddle/fluid/layers/control_flow.py

@@ -2488,9 +2488,6 @@ def _error_message(what, arg_name, op_name, right_value, error_value):
 def case(pred_fn_pairs, default=None, name=None):
     '''
     :api_attr: Static Graph
-	:alias_main: paddle.nn.case
-	:alias: paddle.nn.case,paddle.nn.control_flow.case
-	:old_api: paddle.fluid.layers.case
 
     This operator works like an if-elif-elif-else chain.
 
@@ -2500,7 +2497,7 @@ def case(pred_fn_pairs, default=None, name=None):
         name(str, optional): The default value is None. Normally there is no need for user to set this property. For more information, please refer to :ref:`api_guide_Name`.
 
     Returns:
-        Variable|list(Variable): Tensors returned by the callable from the first pair whose pred is True,
+        Tensor|list(Tensor): Tensors returned by the callable from the first pair whose pred is True,
         or Tensors returned by ``default`` if no pred in ``pred_fn_pairs`` is True and ``default`` is not None,
         or Tensors returned by the last callable in ``pred_fn_pairs``  if no pred in ``pred_fn_pairs`` is True and ``default`` is None.
 
@@ -2508,45 +2505,47 @@ def case(pred_fn_pairs, default=None, name=None):
         TypeError: If the type of ``pred_fn_pairs`` is not list or tuple.
         TypeError: If the type of elements in ``pred_fn_pairs`` is not tuple.
         TypeError: If the size of tuples in ``pred_fn_pairs`` is not 2.
-        TypeError: If the first element of 2-tuple in ``pred_fn_pairs`` is not Variable.
+        TypeError: If the first element of 2-tuple in ``pred_fn_pairs`` is not a Tensor.
         TypeError: If the second element of 2-tuple in ``pred_fn_pairs`` is not callable.
         TypeError: If ``default`` is not None but it is not callable.
 
     Examples:
         .. 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):
-                x = layers.fill_constant(shape=[1], dtype='float32', value=0.3)
-                y = layers.fill_constant(shape=[1], dtype='float32', value=0.1)
-                z = layers.fill_constant(shape=[1], dtype='float32', value=0.2)
+            main_program = paddle.static.default_startup_program()
+            startup_program = paddle.static.default_main_program()
 
-                pred_1 = layers.less_than(z, x)  # true: 0.2 < 0.3
-                pred_2 = layers.less_than(x, y)  # false: 0.3 < 0.1
-                pred_3 = layers.equal(x, y)      # false: 0.3 == 0.1
+            with paddle.static.program_guard(main_program, startup_program):
+                x = paddle.fill_constant(shape=[1], dtype='float32', value=0.3)
+                y = paddle.fill_constant(shape=[1], dtype='float32', value=0.1)
+                z = paddle.fill_constant(shape=[1], dtype='float32', value=0.2)
+
+                pred_1 = paddle.less_than(z, x)  # true: 0.2 < 0.3
+                pred_2 = paddle.less_than(x, y)  # false: 0.3 < 0.1
+                pred_3 = paddle.equal(x, y)      # false: 0.3 == 0.1
 
                 # Call fn_1 because pred_1 is True
-                out_1 = layers.case(
+                out_1 = paddle.static.nn.case(
                     pred_fn_pairs=[(pred_1, fn_1), (pred_2, fn_2)], default=fn_3)
 
                 # Argument default is None and no pred in pred_fn_pairs is True. fn_3 will be called.
                 # because fn_3 is the last callable in pred_fn_pairs.
-                out_2 = layers.case(pred_fn_pairs=[(pred_2, fn_2), (pred_3, fn_3)])
+                out_2 = paddle.static.nn.case(pred_fn_pairs=[(pred_2, fn_2), (pred_3, fn_3)])
 
-                exe = fluid.Executor(fluid.CPUPlace())
+                exe = paddle.static.Executor(paddle.CPUPlace())
                 res_1, res_2 = exe.run(main_program, fetch_list=[out_1, out_2])
                 print(res_1)  # [[1. 1.]]
                 print(res_2)  # [3 3 3]

python/paddle/nn/__init__.py

@@ -38,7 +38,6 @@ from .clip import GradientClipByValue  #DEFINE_ALIAS
 # from .clip import set_gradient_clip        #DEFINE_ALIAS
 from .clip import clip  #DEFINE_ALIAS
 from .clip import clip_by_norm  #DEFINE_ALIAS
-from .control_flow import case  #DEFINE_ALIAS
 from .control_flow import cond  #DEFINE_ALIAS
 # from .control_flow import DynamicRNN        #DEFINE_ALIAS
 # from .control_flow import StaticRNN        #DEFINE_ALIAS

python/paddle/nn/control_flow.py

@@ -13,14 +13,12 @@
 # limitations under the License.
 
 # TODO: define the control flow api  
-from ..fluid.layers import case  #DEFINE_ALIAS
 from ..fluid.layers import cond  #DEFINE_ALIAS
 from ..fluid.layers import while_loop  #DEFINE_ALIAS
 
 from ..fluid.layers import switch_case  #DEFINE_ALIAS
 
 __all__ = [
-    'case',
     'cond',
     #       'DynamicRNN',
     #       'StaticRNN',

python/paddle/static/nn/__init__.py

@@ -17,6 +17,7 @@ __all__ = [
     'batch_norm',
     'embedding',
     'bilinear_tensor_product',
+    'case',
     'conv2d',
     'conv2d_transpose',
     'conv3d',
@@ -39,6 +40,7 @@ __all__ = [
 from ...fluid.layers import fc  #DEFINE_ALIAS
 from ...fluid.layers import batch_norm  #DEFINE_ALIAS
 from ...fluid.layers import bilinear_tensor_product  #DEFINE_ALIAS
+from ...fluid.layers import case  #DEFINE_ALIAS
 from ...fluid.layers import conv2d  #DEFINE_ALIAS
 from ...fluid.layers import conv2d_transpose  #DEFINE_ALIAS
 from ...fluid.layers import conv3d  #DEFINE_ALIAS