fix bug in example codes of API case and switch_case. test=develop,test=document_fix (#21477)

python/paddle/fluid/layers/control_flow.py

@@ -1911,6 +1911,7 @@ def case(pred_fn_pairs, default=None, name=None):
         .. code-block:: python
 
             import paddle.fluid as fluid
+            import paddle.fluid.layers as layers
 
             def fn_1():
                 return layers.fill_constant(shape=[1, 2], dtype='float32', value=1)
@@ -1923,7 +1924,7 @@ def case(pred_fn_pairs, default=None, name=None):
 
             main_program = fluid.default_startup_program()
             startup_program = fluid.default_main_program()
-            with program_guard(main_program, startup_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)
@@ -3014,6 +3015,8 @@ def switch_case(branch_index, branch_fns, default=None, name=None):
         .. code-block:: python
 
             import paddle.fluid as fluid
+            import paddle.fluid.layers as layers
+
             def fn_1():
                 return layers.fill_constant(shape=[1, 2], dtype='float32', value=1)
 
@@ -3025,7 +3028,7 @@ def switch_case(branch_index, branch_fns, default=None, name=None):
 
             main_program = fluid.default_startup_program()
             startup_program = fluid.default_main_program()
-            with program_guard(main_program, startup_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)