Make convert_to_onnx runable as script again
Author: mcernusca
Pull request: yes

Fix `convert_graph_to_onnx.py` script crash by replacing relative import.

# What does this PR do?

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Fixes `python transformers/convert_graph_to_onnx.py` crash with error `Error while converting the model: attempted relative import with no known parent package` . I found this was previously fixed in #10857 and regressed. 

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Comments:
It seems there is an issue with your CircleCI permissions, the tests won't run.
Could you try refreshing your permissions as shown [here](https://support.circleci.com/hc/en-us/articles/360048210711-How-to-Refresh-User-Permissions-)?

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20009). All of your documentation changes will be reflected on that endpoint.

> It seems there is an issue with your CircleCI permissions, the tests won't run. Could you try refreshing your permissions as shown [here](https://support.circleci.com/hc/en-us/articles/360048210711-How-to-Refresh-User-Permissions-)?

Thanks, CircleCI supposedly has access to all my repositories now but I'm not sure how to re-trigger the tests. Sorry if I'm missing something obvious.

You can try an empty commit (`git commit -m "Trigger CI" --allow-empty`)

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.

Sorry it fell under our radar. Don't hesitate to ping me next time it happens!