extract //c10/core:ScalarType to its own library (#70855)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70855
This library is depended on by parts of util so it has to go next.
ghstack-source-id: 147170897
Test Plan: Rely on CI.
Reviewed By: malfet
Differential Revision: D33329527
fbshipit-source-id: 28a111f602ee085c1d9b0acec29790488f8c8f0b
(cherry picked from commit e3601b94ff4a89caeb0c012a0d946613934646b9)
