Add class LayoutLMv2ForRelationExtraction
# What does this PR do?
Add class LayoutLMv2ForRelationExtraction which in https://github.com/microsoft/unilm/tree/master/layoutlmft
I've [implemented](https://github.com/woflowinc/transformers/tree/add-layoutlmv2-re) the requested changes, but I'm not sure how to contribute to this PR from here.<|||||>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.<|||||>Hi @quasimik,

could you open up a clean PR such that we can add it? 