Add OPT architecture

[MartinGleize]

What does this PR do? Please describe:
This PR adds the OPT architecture, from "OPT: Open Pre-trained Transformer Language Models".

Does your PR introduce any breaking changes? If yes, please list them:
No

Check list:

• Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
• Did you read the contributor guideline?
• Did you make sure that your PR does only one thing instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (no need for typos, documentation, or minor internal changes)