replacing ubuntu-latest with ubuntu-18.04 (#51744)
Summary:
following https://github.com/pytorch/pytorch/pull/51725#pullrequestreview-583703598
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51744
Reviewed By: samestep
Differential Revision: D26262089
Pulled By: janeyx99
fbshipit-source-id: fa24e5c15d24750f2a5ccd5b6a5aad9a4a3ad09f
