Occasional Can not load roberta-base tokenizer 
**Environment info**
transformers 4.12.0.dev

**issue**
when using `tokenizer = AutoTokenizer.from_pretrained('roberta-base')`, got the bug as following:
```
OSError: Can't load config for 'roberta-base'. Make sure that:

- 'roberta-base' is a correct model identifier listed on 'https://huggingface.co/models'
  (make sure 'roberta-base' is not a path to a local directory with something else, in that case)

- or 'roberta-base' is the correct path to a directory containing a config.json file
```
Sometimes this bug occurs while sometimes it works well
@LysandreJik

I think this is probably due to an internet connection error, unfortunately<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.