How to export to ONNX distilbert with question-answering head
I am having a little bit of trouble exporting to ONNX: `distilbert` with `question-answering` head.

### Code

I convert to ONNX according to [docs](https://huggingface.co/transformers/serialization.html#converting-an-onnx-model-using-the-transformers-onnx-package):

```shell
# Convert to ONNX
!python -m transformers.onnx --model='mrm8488/distill-bert-base-spanish-wwm-cased-finetuned-spa-squad2-es' --feature question-answering 'onnx_exported'
```

But `--feature question-answering ` seems like is not an acceptable feature.

Is this expected?

### Versions

```
transformers	4.11.2
Python		3.7.12
```

### Inquiry

The reason I am doing this is to run pipeline with `onnx` and get extra juice out of it (performance). Is there a better way than this manual export approach?
I don't think the `question-answering` head is implemented, cc @michaelbenayoun <|||||>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.

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