This reverts commit c2c99dc from #22795.

Hello!

What does this PR do?

Removes Open-Llama, a confusingly named and unused model architecture from which all documentation links throw 404 errors.

Motivation

• The Open-Llama-V1 model and Open-Llama GitHub repository have both been removed.
• There is only one issue on the transformers repo that refers to OpenLlamaForCausalLM.
• There is only one model on the Hub that mentions OpenLlamaForCausalLM.
• All of the ~1900 other LLama models use the standard LlamaForCausalLM instead.
• Users on the HF Discord have been confused by the Open-Llama documentation page.

In case you are opposed out of principle, i.e. that no architectures should be removed, then we may want to at least update the documentation. However, I urge you to remove this confusing architecture.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Reviewers of the original PR: @ArthurZucker @sgugger
cc: @s-JoL

• Tom Aarsen