move checkpoint api to trainer.py

python/paddle/fluid/io.py

@@ -24,8 +24,7 @@ from . import core
 __all__ = [
     'save_vars', 'save_params', 'save_persistables', 'load_vars', 'load_params',
     'load_persistables', 'save_inference_model', 'load_inference_model',
-    'get_inference_program', 'save_checkpoint', 'load_checkpoint',
-    'clean_checkpoint'
+    'get_inference_program'
 ]