Add flag to temporarily enable first class modules (#21560)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21560
ghimport-source-id: a555ca33fcd3efd1147aaf90f26a8e63da1c1a67
Reviewed By: suo
Differential Revision: D15729502
Pulled By: zdevito
fbshipit-source-id: d6c11472bfc791e2ad1e9aa695b0439d72b79681
