Add flag to temporarily enable first class modules (#21272)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21272
ghimport-source-id: 43e73d1b93ccbe0dd6845eb3f7444c9d0abd444b
Reviewed By: jamesr66a
Differential Revision: D15600068
Pulled By: zdevito
fbshipit-source-id: 9b68e23d7f8b6046a5a0d6d9fd16138ac384b863
