Refactor structured set_output in Register{DispatchKey}.cpp (#62188)

Refactor structured set_output in Register{DispatchKey}.cpp (#62188)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62188

These parts of the `set_output` code are identical for all operators in the
kernel registration files. So, this moves them from being copied into every
class to two helper functions at the top of the file.

Test Plan: Imported from OSS

Reviewed By: soulitzer

Differential Revision: D29962045

Pulled By: albanD

fbshipit-source-id: 753b8aac755f3c91b77ffa2c30a89ac91a84b7c4