Add OPT model parallelize
# Model Parallelism for OPT

Added ```parallelize``` and ```deparallelize``` methods on ```OPTDecoder```, ```OPTModel``` and ```OPTForCausalLM```.
Referred to ```gpt2``` model parallelize (https://github.com/huggingface/transformers/pull/8696).

Fixes #17240 

## Who can review?
Let me know if you need any modifications, @patrickvonplaten
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17245). All of your documentation changes will be reflected on that endpoint.<|||||>Hey @lkm2835,

Thanks for your PR - however this way of parallelizing the model is a bit outdated. The recommended way of using the model in parallel is to use `accelerate` see: https://twitter.com/huggingface/status/1524783489593360385

We'll soon have this natively supported in `transformers` as well cc @sgugger <|||||>Then, is it better to close this PR?<|||||>> natively

I'm afraid so! There are lots of other "Good first issues" or "Good second issues" though if you'd like to give it a try :-)<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.