Add onnx export of models with a multiple choice classification head
This PR adds the export support of models with a multiple-choice classification head, resolving [#16695](https://github.com/huggingface/transformers/issues/16695)

This includes the following additions:
* The `"multiple-choice"` feature was added to the corresponding model topologies
* The dummy inputs are generated to match the expected inputs shape which includes an extra dimension corresponding to the number of candidates answers
* The `inputs` method of the models corresponding `OnnxConfig` were modified to support the additional dynamic axis corresponding to the number of candidates

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the  review as well as the reminder @sgugger !<|||||>Thanks for the reviews @lewtun and @michaelbenayoun. 
I have added some comments to make things clearer as well as the BigBird, Data2VecText, Electra and FlauBERT models support. Also when running the command line `RUN_SLOW=1 pytest tests/onnx/test_onnx_v2.py`, all the tests are passing.<|||||>Thanks for iterating on this @echarlaix - it looks great! 