Cannot import RobertaPreTrainedModel
## Environment info
I tried with both transformers 3.5.1 and 4.6.1.

### Who can help
Maybe @julien-c since they contributed RoBERTa.

## Information
I want to derive my own class from RobertaPreTrainedModel, but I cannot import that class like I can import e.g. BertPreTrainedModel or AlbertPreTrainedModel. More specifically,

```from transformers import BertPreTrainedModel```
and
```from transformers import AlbertPreTrainedModel```
works, but
```from transformers import RobertaPreTrainedModel```
returns `ImportError: cannot import name 'RobertaPreTrainedModel'`.

Is this the intended behavior or could it be a bug?

## To reproduce

Try `from transformers import RobertaPreTrainedModel`

## Expected behavior

The RobertaPreTrainedModel class should be imported like it works for other transformers.