[`Llama2`]  Add support for Llama 2
# What does this PR do?
Add Support for Llama2 ! 🔥 🔥 
_The documentation is not available anymore as the PR was closed or merged._<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_24891). All of your documentation changes will be reflected on that endpoint.