Support for OPT Models

[mkirch]

Would be great to figure out how to support OPT models. models.md has a note that OPT is not supported yet:

OPT [TODO verify] Conversion error on a slicing operation.

Bloom still has the same note but is now fully supported by exporters. So I'm wondering if there actually still an issue with the OPT models, or if the underlying issue was resolved already. If so, then they could be listed as supported. Happy to pitch in if anyone has context on outstanding issues with the OPT models.

Thanks!