Remove unsupported and confusing "OpenLlama" architecture
This reverts commit c2c99dc7ef5edab8f7674a1eb00cf6ac6996fd0f from #22795.

Hello!

# What does this PR do?

Removes [Open-Llama](https://huggingface.co/docs/transformers/model_doc/open-llama), a confusingly named and unused model architecture from which all documentation links throw 404 errors. 

## Motivation
* The [Open-Llama-V1 model](https://huggingface.co/s-JoL/Open-Llama-V1) and [Open-Llama GitHub repository](https://github.com/s-JoL/Open-Llama) have both been removed.
* There is only [one issue](https://github.com/huggingface/transformers/issues?q=OpenLlamaForCausalLM) on the transformers repo that refers to `OpenLlamaForCausalLM`.
* There is only [one model](https://huggingface.co/search/full-text?q=OpenLlamaForCausalLM) 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](https://huggingface.co/docs/transformers/model_doc/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).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

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

- Tom Aarsen
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_24913). All of your documentation changes will be reflected on that endpoint.<|||||>No we cannot removing the architecture entirely that would be a breaking change in the library that would be unprecedented and against our philosphy/commitment to backward compatibility.

Even removing it entirely from the docs seem a bit extreme. If users are confused by which class to use to load a model, they should use the Auto model API to let it select the right class for them. We can add a disclaimer in the doc page of open-llama doc or move it to the deprecated models but that's pretty much the extent of what we can do.

cc @LysandreJik <|||||>I had such a fear - I understand your reasoning completely. Whichever way we go, we end up with a suboptimal situation. I'll let the user on Discord know that it won't be changed. 

I could add a disclaimer that the OpenLlama models do not use the OpenLlama architecture, but simply the Llama one.<|||||>> I could add a disclaimer that the OpenLlama models do not use the OpenLlama architecture, but simply the Llama one.

Yes, that would be great!