Enable fused optimizer for DP (#98270)
Differential Revision: [D42714482](https://our.internmc.facebook.com/intern/diff/D42714482/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D42714482/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98270
Approved by: https://github.com/awgu
