Add a util to generate schema for out variant op from other variants (#1253)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1253

As titled, this is helpful for custom ops authoring.

Reviewed By: JacobSzwejbka

Differential Revision: D51490001

fbshipit-source-id: 7fd97e7662ec2eb2fa2900bd4b15af4a8f4445de