[WIP] Implement LayoutLMv2ForRelationExtraction (continues #15173)
# What does this PR do?

Continues the good work in #15173 to add LayoutLMv2ForRelationExtraction, as implemented in [Microsoft's UniLM repo](https://github.com/microsoft/unilm/blob/152193af4b295ae39cf0c2a492da3ee5cc5abe29/layoutlmft/layoutlmft/models/layoutlmv2/modeling_layoutlmv2.py#L895-L937)

Tests are not written yet, and I might need help with that.

@NielsRogge
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_18943). All of your documentation changes will be reflected on that endpoint.<|||||>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/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Hi,

Thanks for your work. However, we don't want to add LayoutLMv2ForRelationExtraction with the design that the Microsoft authors created (as the model returns lists, rather than fixed size tensors). The latter is necessary for the model to work on a distributed environment, and for things like ONNX. See comments at #19120<|||||>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/main/CONTRIBUTING.md) are likely to be ignored.