Polish api BuildStrategy/ExecutionStrategy doc & code example (#27662)

* polish BuildStrategy api doc & example

* polish ExecutionStrategy api doc & example

* polish details

python/paddle/static/__init__.py

@@ -19,7 +19,7 @@ __all__ = [
     'name_scope', 'ParallelExecutor', 'program_guard', 'WeightNormParamAttr',
     'default_main_program', 'default_startup_program', 'Program', 'data',
     'InputSpec', 'save', 'load', 'save_inference_model', 'load_inference_model',
-    'load_program_state', 'set_program_state'
+    'load_program_state', 'set_program_state', 'cpu_places', 'cuda_places'
 ]
 
 from . import nn
@@ -38,6 +38,8 @@ from ..fluid.framework import default_startup_program  #DEFINE_ALIAS
 from ..fluid.framework import Program  #DEFINE_ALIAS
 from ..fluid.framework import name_scope  #DEFINE_ALIAS
 from ..fluid.framework import program_guard  #DEFINE_ALIAS
+from ..fluid.framework import cpu_places  #DEFINE_ALIAS
+from ..fluid.framework import cuda_places  #DEFINE_ALIAS
 from ..fluid.layers.control_flow import Print  #DEFINE_ALIAS
 from ..fluid.layers.nn import py_func  #DEFINE_ALIAS
 from ..fluid.parallel_executor import ParallelExecutor  #DEFINE_ALIAS