Implement RMT-R (New Paper feature to RMTs)

[anoojpatel]

RMT-R or Recurrent Memory Transformer - Retrieval is a new paper from the lab that describes a methodology to inject Past M_t-1 Memories into a Retrieval Cross attention head to improve performance at 10M token lengths. It would be cool to add this feature!

Paper reference: https://arxiv.org/pdf/2402.10790.pdf