Make script::Method a value type (#21675)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21675
ghimport-source-id: 90ee7ba00e58b0151ca4c17e91fd17303c9d5d08
Test Plan: Imported from OSS
Reviewed By: suo
Differential Revision: D15777725
Pulled By: zdevito
fbshipit-source-id: 8482cd2e1dcd7dd77a9cacbb76743bd190c7c4cf
