🌟 New model head addition

Relation Extraction Head for LayoutLMv2/XLM

Addition description

Hey all,

I've see a bunch of different requests across huggingface issues [0], unilm issues [0][1] and on @NielsRogge Transformer Tutorials issues [0][1] about adding the relation extraction head from layoutlmv2 to the huggingface library. As the model is quite difficult to use in it's current state I was going to write my own layer ontop but I saw in this issue that it may be a good idea to add it to transformers as a separate layoutlmv2/xlm head and thought it would be a good way to contribute back to a library I use so much.

I've gone ahead and added it under my own branch and got it successfully working with the library. Here is a colab using my branch of transformers if you want to test it yourself.

Before I add tests/ write more docs I just wanted to post here first to see if there's interest in potentially merging this in. If there is interest I have a few questions that it would be helpful to get some info on to ensure that I've correctly done the integration.