Fix Gemini chat issue. (#4757)

### What problem does this PR solve?

#4753

### Type of change

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

rag/llm/chat_model.py

@@ -1375,7 +1375,7 @@ class GoogleChat(Base):
         from google.oauth2 import service_account
         import base64
 
-        key = json.load(key)
+        key = json.loads(key)
         access_token = json.loads(
             base64.b64decode(key.get("google_service_account_key", ""))
         )