fixed crash when deleting older checkpoint and a file f"{checkpoint_prefix}-*" exist (#16686)

I create an archive of older checkpoints during training the checkpoint has a  name with `f"{checkpoint_prefix}-*.zip/.tar ` 
previously `glob(f"{checkpoint_prefix}-*")` takes all files/folders starting with the name checkpoint, and later `shutil.rmtree(checkpoint)` takes a folder name; since at some point it my get a zip file; it crashes training; adding this `if os.path.isdir(x)` allows only folders on `glob_checkpoints`

src/transformers/trainer.py

@@ -2200,7 +2200,7 @@ class Trainer:
     ) -> List[str]:
         ordering_and_checkpoint_path = []
 
-        glob_checkpoints = [str(x) for x in Path(output_dir).glob(f"{checkpoint_prefix}-*")]
+        glob_checkpoints = [str(x) for x in Path(output_dir).glob(f"{checkpoint_prefix}-*") if os.path.isdir(x)]
 
         for path in glob_checkpoints:
             if use_mtime: