How to use LayoutLMv3 for Relation Extraction?

it seems to be no RelationExtraction head to layoutLMv3

It would be identical to LayoutLMv2.

How would this be done using layoutlm v2?

You can add a bilinear layer on top of the last token representation layer. Check out this paper [2404.10848] A LayoutLMv3-Based Model for Enhanced Relation Extraction in Visually-Rich Documents