Add ResNet to models exportable with ONNX
Author: chamidullinr
Pull request: yes

# What does this PR do?

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I added OnnxConfig to make ResNet model available for ONNX conversion.

Issue [#16308](https://github.com/huggingface/transformers/issues/16308)

## 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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- [ ] Did you write any new necessary tests?


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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:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_16948). All of your documentation changes will be reflected on that endpoint.

Thanks for the suggestions. One test fails when running slow tests. The error is:
> ValueError: Outputs values doesn't match between reference model and ONNX exported model: Got max absolute difference of: 0.000244140625

Should I set --atol=1e-3 or is there some way to fix this?


> Should I set --atol=1e-3 or is there some way to fix this?

Ah yes, some models require a lower tolerance due to their architectures. Setting `--atol=1e-3` as the default is fine!

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.

Can't we rebase this branch and add the code for ResNet ?

I'm taking care of this in #17585, I didn't see this PR before opening mine. @ChainYo

It should be done by the end of the week! 

> It should be done by the end of the week!

Pretty cool!