support relative path for best_model_checkpoint
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Fixes #7431

When I give a relative path to `output_dir`, it raises some error at line 1222.
https://github.com/huggingface/transformers/blob/83481056921296fadbdc86cd51c157a9a9327946/src/transformers/trainer.py#L1205-L1227

If I give './path1/path2' to `output_dir`, `checkpoints_sorted` becomes 'path1/path2' by Path library on line 1208.
But `self.state.best_model_checkpoint` is still './path1/path2'.
So it will raise error as below.
```
Traceback (most recent call last):
  File "../finetune.py", line 369, in <module>
    main(**vars(args))
  File "../finetune.py", line 315, in main
    device)
  File "../finetune.py", line 120, in train
    trainer.train(optim_pretrained_path)
  File "/usr/local/lib/python3.6/dist-packages/transformers/trainer.py", line 803, in train
    self._maybe_log_save_evaluate(tr_loss, model, trial, epoch)
  File "/usr/local/lib/python3.6/dist-packages/transformers/trainer.py", line 860, in _maybe_log_save_evaluate
    self._save_checkpoint(model, trial, metrics=metrics)
  File "/usr/local/lib/python3.6/dist-packages/transformers/trainer.py", line 918, in _save_checkpoint
    self._rotate_checkpoints(use_mtime=True)
  File "/usr/local/lib/python3.6/dist-packages/transformers/trainer.py", line 1235, in _rotate_checkpoints
    checkpoints_sorted = self._sorted_checkpoints(use_mtime=use_mtime)
  File "/usr/local/lib/python3.6/dist-packages/transformers/trainer.py", line 1223, in _sorted_checkpoints
    best_model_index = checkpoints_sorted.index(self.state.best_model_checkpoint)
ValueError: './results/use_pretrained_test/checkpoint-1162' is not in list
```

So I resolve this error by using Path for `self.state.best_model_checkpoint`.
Any other idea is also welcome. please check it
