Add codereviewer
Author: fasterinnerlooper
Pull request: yes

# What does this PR do?

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Fixes #29490 


## 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#create-a-pull-request),
      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.
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      [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).
- [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.

<!-- Your PR will be replied to more quickly if you can figure out the right person to tag with @

 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.

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: @muellerzr and @pacman100

Integrations:

- deepspeed: HF Trainer/Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization (bitsandbytes, autogpt): @SunMarc and @younesbelkada

Documentation: @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: See Models above and tag the person corresponding to the modality of the example.
- TensorFlow: @Rocketknight1

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Comments:
Hi all.

I'm hoping someone like @mariosasko or @lvwerra can guide me through the rest of the process. I've never committed to a repo this big before and I know I've probably missed a few things and gotten a few things wrong so any assistance on this would be greatly appreciated.
The aim of this PR is to add the https://huggingface.co/microsoft/codereviewer model to Transformers so that it can be easily used and fine-tuned by others. I don't work for Microsoft and I don't have any affiliation with them but I feel like this is a really good model to use as the basis for further fine-tuning and for expanding on.
There are a few things that I haven't implemented because I wanted to get this model added before working on some of the other tasks. It's taken me about a month of on-the-side work to get this far so please be gentle!

The part of the model that I have implemented is the Seq2Seq portion (thought I have used to Conditional Generation head), which takes a diff and outputs a relevant comment.

The things I haven't implemented are:
The <CLS> token is added to the output to show if the diff is of good quality. This would be a good fit for a binary classification task, so I think it might be best to do some training for that task and then add that to the model's save files once it's done (though suggestions and recommendations around this are appreciated).
The other task that the base model does is write PR comments when given a diff. I think this might be better for a Text Generation task, and again, would be something that I would work on once this PR has been merged. 

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.