Fix type errors: declare _forked_checkpoint_dir on UnslothService

Author: arcticfly

src/art/local/backend.py

@@ -1451,7 +1451,7 @@ async def _experimental_fork_checkpoint(
                 print(
                     "Invalidated UnslothService _state cache to pick up forked checkpoint"
                 )
-        service._forked_checkpoint_dir = dest_checkpoint_dir
+        service._forked_checkpoint_dir = dest_checkpoint_dir  # type: ignore[union-attr]
 
         if verbose:
             print(

src/art/unsloth/service.py

@@ -111,6 +111,7 @@ class UnslothService:
     output_dir: str
     _is_sleeping: bool = False
     _latest_step: int = 0
+    _forked_checkpoint_dir: str | None = None
     _lora_id_counter: int = 1  # Start from 1 since 0 is reserved
     # Dedicated mode subprocess state
     _vllm_process: subprocess.Popen | None = field(default=None, repr=False)  # type: ignore[type-arg]