defining optional

exo/api/chatgpt_api.py

@@ -17,7 +17,7 @@ from exo.helpers import PrefixDict, shutdown
 from exo.inference.tokenizers import resolve_tokenizer
 from exo.orchestration import Node
 from exo.models import build_base_shard, model_cards, get_repo, pretty_name
-from typing import Callable
+from typing import Callable, Optional
 from exo.download.hf.hf_shard_download import HFShardDownloader
 
 class Message: