Update the documentation for AMP with DataParallel (#69218)
Summary:
Following https://github.com/pytorch/pytorch/issues/60540 and pull request https://github.com/pytorch/pytorch/issues/43102
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69218
Reviewed By: gchanan
Differential Revision: D32803814
Pulled By: ngimel
fbshipit-source-id: 06fdbbee2c7734153271be70ec4bc24263c8c367
