Update custom_st.py to be compatible with Python 3.8

Use the Dict[str, Any] syntax instead of dict[str, Any] for get_config_dict()

custom_st.py

@@ -190,7 +190,7 @@ class Transformer(nn.Module):
         )
         return output
 
-    def get_config_dict(self) -> dict[str, Any]:
+    def get_config_dict(self) -> Dict[str, Any]:
          return {key: self.__dict__[key] for key in self.config_keys}
 
     def save(self, output_path: str, safe_serialization: bool = True) -> None: