Introduce SerializableType (#21835)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21835
ghimport-source-id: e674048a56b9a573ba89e484f4b41818d3f08234
Test Plan: Imported from OSS
Reviewed By: zdevito
Differential Revision: D15860004
Pulled By: jamesr66a
fbshipit-source-id: 2d2905296939903ed4586932bea0a504b542bbdb
