Add config for generating ONNX models for table-transformers
Author: antew
Pull request: yes

# What does this PR do?

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Allows generating ONNX models for table-transformers.

## 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#start-contributing-pull-requests),
      Pull Request section?
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      to it if that's the case.
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      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
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- [ ] Did you write any new necessary tests?


## Who can review?

@NielsRogge - This adds a small bit on top of the work you did in https://github.com/huggingface/transformers/pull/19614 to allow generating ONNX models for table transfomers (e.g. `python -m transformers.onnx --model="microsoft/table-transformer-structure-recognition" table/`)

Without this addition, I was getting this error:

```
KeyError: "table-transformer is not supported yet.
Only ['albert', 'bart', 'beit', 'bert', 'big-bird', 'bigbird-pegasus', 'blenderbot', 'blenderbot-small', 'bloom', 'camembert', 'clip', 'codegen', 'convbert', 'convnext', 'data2vec-text', 'data2vec-vision', 'deberta', 'deberta-v2', 'deit', 'detr', 'distilbert', 'electra', 'flaubert', 'gpt2', 'gptj', 'gpt-neo', 'groupvit', 'ibert', 'imagegpt', 'layoutlm', 'layoutlmv3', 'levit', 'longt5', 'longformer', 'marian', 'mbart', 'mobilebert', 'mobilenet_v1', 'mobilenet_v2', 'mobilevit', 'mt5', 'm2m-100', 'owlvit', 'perceiver', 'resnet', 'roberta', 'roformer', 'segformer', 'squeezebert', 'swin', 't5', 'vision-encoder-decoder', 'vit', 'whisper', 'xlm', 'xlm-roberta', 'yolos'] 
are supported. If you want to support table-transformer please propose a PR or open up an issue.
```

Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20744). All of your documentation changes will be reflected on that endpoint.

Note that we don't accept new model exports in Transformers, as we moved that part into the Optimum library. You should open a PR to add support there :-)

> Note that we don't accept new model exports in Transformers, as we moved that part into the Optimum library. You should open a PR to add support there :-)

Thank you @sgugger, will do!