Export defs.bzl to open source for pytorch (#15132)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15132
Pull Request resolved: https://github.com/facebook/fbshipit/pull/64
Reviewed By: dzhulgakov
Differential Revision: D13424093
fbshipit-source-id: bbebef964b9f3aef8f59cd394eca068680c36b5a
