support ONNX export of XDropout in deberta{,_v2} and sew_d
# What does this PR do?

Enables `torch.onnx.export` of the `StableDropout` module in training mode.
In training mode, the `XDropout` torch.autograd.Function is included. This change
adds a `symbolic` function to `XDropout` which produces an ONNX graph that
is equivalent to the `forward` function.


## Before submitting

- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/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.
No. LMK if you want me to open an issue.

- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).

No, LMK if there's some doc that I should update.

- [x] Did you write any new necessary tests?

## Who can review?

@LysandreJik

_The documentation is not available anymore as the PR was closed or merged._<|||||>@michaelbenayoun thanks for taking a look. I added a test.
Unfortunately I couldn't use the existing testing style because none of the affected models have an ONNX config (that's tracked by https://github.com/huggingface/transformers/issues/16308).<|||||>@michaelbenayoun bump, PTAL<|||||>@michaelbenayoun bump<|||||>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.<|||||>@garymm any update on this? <|||||>@michaelbenayoun @lewtun do you have any updates on this PR?<|||||>Thank you for your contribution