fix- raise io error when user load from non-existed dir test=develop (#19384)

This PR fix problem with issue #18096 , which raise an error for user to specify the error about load dir is wrong

python/paddle/fluid/dygraph/checkpoint.py

@@ -174,6 +174,9 @@ def _save_var_to_file(stat_dict, optimizers, file_dir, file_name):
 
 
 def _load_var_from_file(file_dir):
+    if not os.path.exists(file_dir):
+        raise IOError("{} not exist".format(file_dir))
+
     def walk_filename(file_dir):
         base_path = os.path.join(file_dir)
         var_name_list = []

python/paddle/fluid/tests/unittests/test_imperative_checkpoint.py

@@ -150,6 +150,10 @@ class TestDygraphCheckpoint(unittest.TestCase):
                         dy_param_init_value[param.name] = param.numpy()
 
                     restore, _ = fluid.dygraph.load_persistables("save_dir")
+
+                    self.assertRaises(IOError, fluid.dygraph.load_persistables,
+                                      "not_exist_dir")
+
                     mnist.load_dict(restore)
 
                     self.assertEqual(len(dy_param_init_value), len(restore))