RetNet model support
### Model description

RetNet / Retentive Networks is a new model *archetype* released by microsoft; the research paper is [here](https://arxiv.org/pdf/2307.08621.pdf). As of now, there is *one* model for retnet; [made by me](https://huggingface.co/parsee-mizuhashi/retnet-tiny-wikitext-undertrained); which is undertrained (`loss=8`!) and I am trying to make a second model on a larger arch.

### Open source status

- [X] The model implementation is available
- [X] The model weights are available

### Provide useful links for the implementation

[commit that has retnet training](https://github.com/microsoft/torchscale/commit/bf65397b26469ac9c24d83a9b779b285c1ec640b)
@donglixp was the main author for commit and cited on the paper
all code is licensed under MIT, including model weights
cc @ArthurZucker @younesbelkada <|||||>p.s. if google offered any bigger TPU's for TRC; i could train retnet-3b (the point at which retnet is better than regular transformers), but as of now; theres retnet_base (small) and retnet_medium (ill upload it when it gets good)<|||||>I am wondering if the original authors released the trained models?<|||||>as far as i know, no official pretrained models were released by microsoft; but the training code is on the torchscale repo, so thats how i am training the models<|||||>Cool model! But as long as we don't have official/ very good pretraining checkpoints, not really anything we can do! <|||||>ah, understood, i'll try to get a good checkpoint; but for now, i assume i can close this and reopen when it finishes training<|||||>oops