Added OnnxConfig for MPNet models
# What does this PR do?

This PR Adds `OnnxConfig` for MPNet based models. 

In order for the conversion to be compatible with some older versions of PyTorch (In my case 1.11.0), I had to add `*args`, to the signature of `forward` functions that were using `**kwargs`. This was because `_decide_input_format` in PyTorch considered `**kwargs` as a normal parameter so it added an additional (unexpected) positional argument (with value None).

https://github.com/pytorch/pytorch/blob/33bb8ae350611760139457b85842b1d7edf9aa11/torch/onnx/utils.py#L793

## 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?
- [x] 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. #16308 
- [x] 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).
- [x] Did you write any new necessary tests?


## Who can review?

@ChainYo 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19009). All of your documentation changes will be reflected on that endpoint.<|||||>@ChainYo As I mentioned in the pr description, a function inside torch (with version 1.11) had an issue parsing the signature correctly and provided kwargs as an extra positional argument. Latest version worked though, but I added the fix for compatibility.<|||||>> @ChainYo As I mentioned in the pr description, a function inside torch (with version 1.11) had an issue parsing the signature correctly and provided kwargs as an extra positional argument. The latest version worked, though, but I added the fix for compatibility.

Sorry I didn't read the PR carefully. Let's see if it doesn't bother the original implementation by bringing any unexpected behaviors. It's okay, then. :hugs: <|||||>Is there anything I can do to help with this?<|||||>> Is there anything I can do to help with this?

We are waiting for a reviewer to get feedback and see what's next!<|||||>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.