pin numpy version to 1.18.5 (#42670)
Summary:
Using numpy 1.19.x instead of 1.18.x breaks certain unit tests.
Fixes https://github.com/pytorch/pytorch/issues/42561. Likely also fixes https://github.com/pytorch/pytorch/issues/42583.
CC ezyang xw285cornell sunway513
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42670
Reviewed By: ezyang
Differential Revision: D22978369
Pulled By: malfet
fbshipit-source-id: ce1f35c7ba620c2b9dd10613f39354cebee8b87d
