Fix bug in _sorted_checkpoints
Author: shaie
Pull request: yes

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
```

# What does this PR do?

<!--
Congratulations! You've made it this far! You're not quite done yet though.

Once merged, your PR is going to appear in the release notes with the title you set, so make sure it's a great title that fully reflects the extent of your awesome contribution.

Then, please replace this with a description of the change and which issue is fixed (if applicable). Please also include relevant motivation and context. List any dependencies (if any) that are required for this change.

Once you're done, someone will review your PR shortly (see the section "Who can review?" below to tag some potential reviewers). They may suggest changes to make the code even better. If no one reviewed your PR after a week has passed, don't hesitate to post a new comment @-mentioning the same persons---sometimes notifications get lost.
-->

<!-- Remove if not applicable -->

Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dimiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests), 
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to the it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests? 


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.

<!-- Your PR will be replied to more quickly if you can figure out the right person to tag with @

 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.

 albert, bert, XLM: @LysandreJik 
 GPT2: @LysandreJik, @patrickvonplaten
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Benchmarks: @patrickvonplaten
 Model Cards: @julien-c
 Translation: @sshleifer
 Summarization: @sshleifer
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
 rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)
 Text Generation: @patrickvonplaten, @TevenLeScao
 Blenderbot, Bart, Marian, Pegasus: @sshleifer
 T5: @patrickvonplaten
 Rag: @patrickvonplaten, @lhoestq
 EncoderDecoder: @patrickvonplaten
 Longformer, Reformer: @patrickvonplaten
 TransfoXL, XLNet: @TevenLeScao, @patrickvonplaten
 examples/seq2seq: @sshleifer
 examples/bert-loses-patience: @JetRunner
 tensorflow: @jplu
 examples/token-classification: @stefan-it
 documentation: @sgugger
 -->


Comments:
Ooh, that's an ugly typo. Thanks for fixing! Will make sure to add a test of this soon so it doesn't break with me being stupid like that.