Add support for `device_map="auto"` to OPT
# What does this PR do?

I forgot to have OPT in the initial list of models supporting `device_map="auto"` (along side GPT-J, GPT-2 and T5). This PR takes care of it.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17382). All of your documentation changes will be reflected on that endpoint.