Added Beit model ouput class
Author: lumliolum
Pull request: yes

# What does this PR do?

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Fixes https://discuss.huggingface.co/t/issue-in-the-documentation-of-transformers-for-biet/10977/2


## Before submitting
- [x] 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](https://github.com/huggingface/transformers/blob/master/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.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@NielsRogge 

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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Comments:
Thanks for your PR! To fix the code quality, you can run make fixup locally (which will run `make style` and `make quality` in sequence).

Hello,

I ran `make html SPHINXOPTS="-W -j 4"` in `docs` directory. This is the error I am getting

```
.virtualenvs/transformers_dev/lib/python3.7/site-packages/recommonmark/parser.py:75: UserWarning: Container node skipped: type=document
  warn("Container node skipped: type={0}".format(mdnode.t))

Warning, treated as an error:
missing attribute forward in object transformers.TapasModel
Makefile:19: recipe for target 'html' failed
make: *** [html] Error 2
```

Note: I ran the above command because in `Circle CI` I saw this
![image](https://user-images.githubusercontent.com/28287182/139691276-196f327c-cab3-4016-b15d-b1c853646448.png)


I've opened a PR on your branch [here](https://github.com/lumliolum/transformers/pull/1).

Thanks for adding this!