Fix bug in _sorted_checkpoints (#7880)

I'm using transformers 3.3.1 and run a training script with `--save_total_limit 3`. I hit the exception below, and after debugging the code found that it wrongly tries to index into the `best_model_checkpoint`'s *str* rather than the `sorted_checkpoints` array. When running without the fix I got this exception:

```
Traceback (most recent call last):
  File "/<HOME>/.conda/envs/transformers/lib/python3.7/site-packages/transformers/trainer.py", line 921, in _save_training
    self._rotate_checkpoints(use_mtime=True)
  File "/<HOME>/.conda/envs/transformers/lib/python3.7/site-packages/transformers/trainer.py", line 1283, in _rotate_checkpoints
    checkpoints_sorted = self._sorted_checkpoints(use_mtime=use_mtime)
  File "/<HOME>/.conda/envs/transformers/lib/python3.7/site-packages/transformers/trainer.py", line 1274, in _sorted_checkpoints
    checkpoints_sorted[best_model_index],
TypeError: 'str' object does not support item assignment
```

src/transformers/trainer.py

@@ -1204,7 +1204,7 @@ class Trainer:
         # Make sure we don't delete the best model.
         if self.state.best_model_checkpoint is not None:
             best_model_index = checkpoints_sorted.index(self.state.best_model_checkpoint)
-            checkpoints_sorted[best_model_index], checkpoints_sorted[best_model_index][-1] = (
+            checkpoints_sorted[best_model_index], checkpoints_sorted[-1] = (
                 checkpoints_sorted[-1],
                 checkpoints_sorted[best_model_index],
             )