[BT] add BetterTransformer support for ProphetNet

[hirotasoshu]

What does this PR do?

Fixes #488 and huggingface/transformers#20372

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?

[hirotasoshu]

With hf-internal-testing/tiny-random-prophetnet and hf-internal-testing/tiny-random-ProphetNetModel models in tests i got this error:

That's why i used microsoft/prophetnet-large-uncased.
Let me know if i need to replace it with some tiny model with bigger max_position_embeddings

[hirotasoshu]

I figured out how to change tiny-random-prophetnet model to make the tests work and upload it to huggingface. Now in the tests it is used

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[fxmarty]

LGTM thank you for the addition!