Fix type: cast Model to TrainableModel for _get_service

Author: arcticfly

src/art/local/backend.py

@@ -1439,7 +1439,7 @@ async def _experimental_fork_checkpoint(
         # _state is a cached_property that reads get_last_checkpoint_dir() on
         # first access; if it was accessed before the fork, it cached the base
         # model and will never pick up the forked weights.
-        service = await self._get_service(model)
+        service = await self._get_service(cast(TrainableModel, model))
         if hasattr(service, "_state") and "_state" in service.__dict__:
             del service.__dict__["_state"]
             if verbose: