Allow passing string to `save_tokenizer_config`

See https://huggingface.co/openGPT-X/Teuken-7B-instruct-research-v0.4/discussions/7.
Need to implement this "upstream" in the tokenizer repo as well.

gptx_tokenizer.py

@@ -395,7 +395,9 @@ class HFGPTXTokenizer(PreTrainedTokenizer):
         old_tokenizer_file_path.replace(new_tokenizer_file_path)
         return str(new_tokenizer_file_path)
     
-    def save_tokenizer_config(self, save_dir: Path) -> None:
+    def save_tokenizer_config(self, save_dir: Union[str, Path]) -> None:
+        save_dir = Path(save_dir)
+
         # convert Path to str
         for k in self.tokenizer_config:
             if isinstance(self.tokenizer_config[k], Path):