Rename prim::Undefined to prim::AutogradZero (#17611)
Summary:
supersedes #17245
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17611
Differential Revision: D14283581
Pulled By: wanchaol
fbshipit-source-id: 8022d02b8a021ea2fee9a18a2c8920eb123200c5
