Add OnnxConfig for ConvBERT
# What does this PR do?

I have added OnnxConfig for ConvBERT model. I have checked all features available (which are the same as DistilBERT), so the features list seems to be good.

## Who can review?

Models:

- albert, bert, xlm: @LysandreJik

_The documentation is not available anymore as the PR was closed or merged._<|||||>The conversion is working as demonstrated by the fact I successfully converted a base model and uploaded it on the hub.

Check the model [here](https://huggingface.co/OWG/convbert-base-spanish)

Here is the command I have used:

```bash
$ python -m transformers.onnx --model=mrm8488/convbert-base-spanish --feature=default onnx/

> Validating ONNX model...
> 	-[✓] ONNX model output names match reference model ({'last_hidden_state'})
> 	- Validating ONNX Model output "last_hidden_state":
>		-[✓] (2, 8, 768) matches (2, 8, 768)
>		-[✓] all values close (atol: 1e-05)
> All good, model saved at: onnx/model.onnx
```<|||||>> The conversion is working as demonstrated by the fact I successfully converted a base model and uploaded it on the hub.
> 
> Check the model [here](https://huggingface.co/OWG/convbert-base-spanish)

Awesome job on pushing this to the Hub! We have a PR in `optimum` coming soon that will allow you to also download these checkpoints directly as ONNX model objects: https://github.com/huggingface/optimum/pull/113<|||||>> Awesome job on pushing this to the Hub! We have a PR in `optimum` coming soon that will allow you to also download these checkpoints directly as ONNX model objects: [huggingface/optimum#113](https://github.com/huggingface/optimum/pull/113)

I dreamed about this and wanted to give it a shot if I had time (spoiler: not in the past months) but someone did it!!!

Btw the tests seem to pass!<|||||>I saw on another onnx config contribution that I could add the model to onnx tests, is it mandatory ? 
Check : #16887<|||||>> I saw on another onnx config contribution that I could add the model to onnx tests, is it mandatory ? Check : #16887

Ah yes, well spotted! You can pick one of the checkpoints from here and add it to the test: https://huggingface.co/models?search=convbert

This will ensure this export is tested as part of our daily CI with the slow tests :)<|||||>Thanks for including the test case - looks great!