Added BERT pretraining example running on Graphcore IPUs to research projects
# What does this PR do?

Adds BERT pretraining example running on Graphcore IPUs to research projects


## Before submitting

- This was discussed in the HuggingFace/Graphcore meeting


## Who can review?

@LysandreJik

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.