Update unpack_dual to return named tuple (#68062)
Summary:
Also updates the doc
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68062
Reviewed By: gchanan
Differential Revision: D32315089
Pulled By: soulitzer
fbshipit-source-id: 567c812da093daeb6549b0dc7ecbffd58eb8ccc2
