Add LTG-BERT model
# LTG-BERT

This pull request adds the custom LTG-BERT model into the repository. This optimized LM architecture was introduced [in this paper](https://arxiv.org/abs/2303.09859) and is currently also used by a new generation of Norwegian LMs. The architecture features multiple improvements to the standard transformer module and we unfortunately cannot use any existing HF model wrappers. 

@ArthurZucker and @younesbelkada
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23037). All of your documentation changes will be reflected on that endpoint.<|||||>Hey! Great that you want to share this model 🔥 Would you be open to put ot on the hub following [this tutorial](https://huggingface.co/docs/transformers/custom_models)! Will be easier as there won't be any CI issues and since it is very similar to an existing model, this makes more sense!<|||||>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.<|||||>Hey if you added the model on the hub could you share the links to it? This will help us keep track of the models that we support on the hub 