Add model UniTE to huggingface repository.
# What does this PR do?

This PR adds model UniTE to the repository of Huggingface.
Paper: [UniTE: Unified Translation Evaluation](https://arxiv.org/abs/2204.13346).
Former discussion is [here](https://github.com/huggingface/transformers/issues/16366).

## Who can review?

@LysandreJik
Anyone in the community is free to review the PR once the tests have passed.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17071). All of your documentation changes will be reflected on that endpoint.<|||||>Thank you for your PR. If I understand correctly, this adds a new type of sequence classification on XLMRoberta, but this won't work at all inside Transformers, since you are not following the API of other models:
- the model should return a `ModelOuput` 
- it should accept the same arguments as other sequence classification models and return the same kind of outputs otherwise it just won't work with the `Trainer` or the `pipeline` function.

Also please follow the general guidelines for a [new model addition](https://huggingface.co/docs/transformers/add_new_model).

<|||||>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.