Fix bugs in API (#3103)

### What problem does this PR solve?

Fix bugs in API


- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

sdk/python/ragflow/modules/chat.py

@@ -68,7 +68,7 @@ class Chat(Base):
             return result_list
         raise Exception(res["message"])
 
-    def delete_sessions(self,ids):
+    def delete_sessions(self,ids:List[str]=None):
         res = self.rm(f"/chats/{self.id}/sessions", {"ids": ids})
         res = res.json()
         if res.get("code") != 0: