Revert "Try to change API"

This reverts commit 36d9e4be.

python/paddle/fluid/io.py

@@ -27,14 +27,10 @@ __all__ = [
     'get_inference_program', 'save_checkpoint', 'load_checkpoint',
     'clean_checkpoint', 'load_persist_vars_without_grad',
     'load_lookup_table_vars', 'save_persist_vars_without_grad',
-    'get_latest_checkpoint_serial', 'foo'
+    'get_latest_checkpoint_serial'
 ]
 
 
-def foo():
-    pass
-
-
 def is_parameter(var):
     """
     Check whether the given variable is an instance of Parameter.