Add wav2vec2-bert ONNX export

[xenova]

What does this PR do?

Fixes # (issue)

@fxmarty I couldn't find a tiny model to test with - once we have that, this should be ready to merge :)

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Did you make sure to update the documentation with your changes?
• [ ] Did you write any new necessary tests?