Opt in flax and tf
# What does this PR do?

Adds support for OPT in both Flax and TF

## Who can review?

@patrickvonplaten, @LysandreJik @younesbelkada @patil-suraj @sgugger 

_The documentation is not available anymore as the PR was closed or merged._<|||||>Should we close the other PR? Let me know once it's ready for a review :-)<|||||>Superseeds https://github.com/huggingface/transformers/pull/17227 and https://github.com/huggingface/transformers/pull/17226<|||||>Cool, very nice job @ArthurZucker ! 

Could you as a final safety guard also add TFOPT and FlaxOPT to the documentation test suite? 

See: https://github.com/huggingface/transformers/tree/main/docs#docstring-testing<|||||>Can I merge @LysandreJik @sgugger ? (failing test are not related to OPT) <|||||>@patil-suraj could you quickly check Flax and maybe @gante go over TF OPT?<|||||>Thanks all for the reviews 😄 🥳 