DeepMind Retro

# 🌟 New model addition

Basically a retrieval augmented model like RAG, but without expensive retriever end2end training

## Open source status

* [ ] the model implementation is available: Not available
* [ ] the model weights are available: Not available
* [ ] who are the authors:(https://arxiv.org/pdf/2112.04426.pdf)

Start by reading the linked DeepMind Retro paper to understand the proposed model and its requirements. The issue provides no implementation files or tests; done would require determining the model scope and making its implementation and weights available, as indicated by the unchecked status items.

Written by the indexing model from the issue text.