Intel OpenVINO backend (inference only)
Author: dkurt
Pull request: yes

# What does this PR do?

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* This PR adds an optional [OpenVINO](https://github.com/openvinotoolkit/openvino) backend which allows to run deep learning inference (not training) on Intel Hardware (CPUs, GPUs, VPUs and other).
* OpenVINO can load models in ONNX or OpenVINO IR formats. Using one of the following classes, conversion is done in runtime (with `from_pt` or `from_tf` flags, see the tests for details).
```
OVAutoModel
OVAutoModelForMaskedLM
OVAutoModelForQuestionAnswering
OVAutoModelWithLMHead
OVAutoModelForSequenceClassification
```
* Users might upload IR format directly to the hub. In example, https://huggingface.co/dkurt/bert-large-uncased-whole-word-masking-squad-int8-0001

forum: https://discuss.huggingface.co/t/intel-openvino-backend/11178  
resolves https://github.com/huggingface/transformers/issues/13987

## 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/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).
- [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:

- 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:
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/master/CONTRIBUTING.md) are likely to be ignored.

Hello @dkurt, as you have seen with @mfuntowicz and @echarlaix, and as you have opened a PR in optimum already, I'll go ahead and close this. It will still be visible and the code will still be accessible, in case reopening it in the future makes sense.

Thank you for your contribution!