Add LlamaForSequenceClassification
Author: lewtun
Pull request: yes

# What does this PR do?

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This PR adds the `LlamaForSequenceClassification` class, which among standard applications can be used for reward modelling in the RLHF pipeline :)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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- [x] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @sgugger

Integrations:

- deepspeed: HF Trainer: @stas00, Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam

Documentation: @sgugger, @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: @sgugger
- TensorFlow: @Rocketknight1

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cc @ArthurZucker and @younesbelkada


Comments:
_The documentation is not available anymore as the PR was closed or merged._

> I would also add potentially a test with single_label_classification to make sure everything works!

Done in https://github.com/huggingface/transformers/pull/22209/commits/6737e380fc6a4cb73150da4fa821dd463f9a7204 :)


Awesome thanks a lot @lewtun ! 🚀