Enable ONNX support for multiple-choice classification heads
# 🚀 Feature request

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Currently, the `transformers.onnx` package doesn't support the export of models with a multiple-choice classification head, i.e. all `ModelForMultiplChoice` classes. We should enable this to provide full coverage of our current exports.

Implementing this involves:

* Adding a `multiple-choice` feature to the `FeaturesManager`
* Generating the appropriate dummy inputs
* Updating the features of all existing models which have a corresponding head

cc @michaelbenayoun 

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