Add either type (#19285)

Add either type (#19285)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19285

The either type is a tagged union with two members.
This is going to be used in a diff stacked on top to allow a function to return one of two types.

Also, generally, either<Error, Result> is a great pattern for returning value_or_error from a function without using exceptions and we could use this class for that later.

Reviewed By: dzhulgakov

Differential Revision: D14931923

fbshipit-source-id: 7d1dd77b3e5b655f331444394dcdeab24772ab3a