Unused Union should not be imported

src/transformers/trainer.py

@@ -7,7 +7,7 @@ import re
 import shutil
 from contextlib import contextmanager
 from pathlib import Path
-from typing import Callable, Dict, List, Optional, Tuple, Union
+from typing import Callable, Dict, List, Optional, Tuple
 
 import numpy as np
 import torch