Add LayoutLMv2 to models exportable with ONNX
This PR adds the code for converting LayoutLMv2 model to onnx format. 
@michaelbenayoun  Can you please review this PR and highlight why these circleci tests are failing ?<|||||>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.<|||||>Hi! Thanks for working on this @fadi212 🙏 I am interested in this work, but I see it has been quite some time since you've had an opportunity to work on it. Are you still working on this? If you no longer have the time or resources to do so, would you be able to provide any next steps or advice on what is necessary for completion? Thank you for your time and effort 🤗 