Adds support for OPT in Flax and TF. 
# What does this PR do?

Adds support for OPT in Flax and TF. 
Also clean Pytorch code a bit. 

## Who can review?
 @LysandreJik, @patrickvonplaten, @patil-suraj, @sgugger


Sorry for the two pull requests in a row, pulled from main instead of rebasing and had the entire commit history. Created a new branch to clean a bit. 
> Thanks for adding those! Without the `TFOPTForCausalLM`, I don't see the point of adding the TF version of OPT since it can't really be used, so would either not add TF yet or make sure this model is added before merging the PR.

Yes I am not done yet! Sorry if I pinged you a bit early<|||||>@ArthurZucker let me know if the PR is ready for a review or you need help with the tests :-) <|||||>> @ArthurZucker let me know if the PR is ready for a review or you need help with the tests :-)

I just have 1 last test that behave strangely (its more about padding tokens and positional embedings) but the jax code will be ready for review tomorrow 12am. Then will work quickly on the tf code and the PR should be ready by the end of the week! <|||||>FLAX code is pretty much done. The only test that I can't solve is the difference in output for the jited model generation! 
<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17227). All of your documentation changes will be reflected on that endpoint.<|||||>@ArthurZucker could we add a test similar to this one: https://github.com/huggingface/transformers/pull/17359 to both Flax and TF? 

@Rocketknight1 @gante could you check the TF version here as well? <|||||>@ArthurZucker,

Do you think we could fix the PR (I think the PR history is a bit messed up). Also totally fine to close this PR and just open a new PR (move all the relevant files to a new PR) if the git correction is too difficult<|||||>> @ArthurZucker,
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> 
> Do you think we could fix the PR (I think the PR history is a bit messed up). Also totally fine to close this PR and just open a new PR (move all the relevant files to a new PR) if the git correction is too difficult

Hey, I think we can close it. 
Will create a new clean branch