save_pretrained: mkdir(exist_ok=True) (#5258)

* all save_pretrained methods mkdir if not os.path.exists

src/transformers/configuration_utils.py

@@ -132,10 +132,9 @@ class PretrainedConfig(object):
             save_directory (:obj:`string`):
                 Directory where the configuration JSON file will be saved.
         """
-        assert os.path.isdir(
-            save_directory
-        ), "Saving path should be a directory where the model and configuration can be saved"
-
+        if os.path.isfile(save_directory):
+            raise AssertionError("Provided path ({}) should be a directory, not a file".format(save_directory))
+        os.makedirs(save_directory, exist_ok=True)
         # If we save using the predefined names, we can load using `from_pretrained`
         output_config_file = os.path.join(save_directory, CONFIG_NAME)