Add missing parentheses in regex expression (#567)

Summary:
The regex pattern without parentheses is not correct. The checkpoints are not sorted in descending order
Pull Request resolved: https://github.com/pytorch/fairseq/pull/567

Differential Revision: D14404380

Pulled By: myleott

fbshipit-source-id: 98cd0cfa8c92b78a03ffbb94840bc0f7a118eca1

train.py

@@ -325,7 +325,7 @@ def save_checkpoint(args, trainer, epoch_itr, val_loss):
 
     if args.keep_last_epochs > 0:
         # remove old epoch checkpoints; checkpoints are sorted in descending order
-        checkpoints = utils.checkpoint_paths(args.save_dir, pattern=r'checkpoint\d+\.pt')
+        checkpoints = utils.checkpoint_paths(args.save_dir, pattern=r'checkpoint(\d+)\.pt')
         for old_chk in checkpoints[args.keep_last_epochs:]:
             if os.path.lexists(old_chk):
                 os.remove(old_chk)