Fix type hint regression (#3033)

Not sure what happened here but some of the file got overwritten by
#2859 which broke filtering logic.

Here is it fixed back to normal.

@hwchase17 can we expedite this if possible :-)

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Co-authored-by: Altay Sansal <altay.sansal@tgs.com>

langchain/retrievers/chatgpt_plugin_retriever.py

@@ -13,7 +13,7 @@ class ChatGPTPluginRetriever(BaseRetriever, BaseModel):
     url: str
     bearer_token: str
     top_k: int = 3
-    filter: Optional[None] = None
+    filter: Optional[dict] = None
     aiosession: Optional[aiohttp.ClientSession] = None
 
     class Config: