Roll master to 1.4.0 (#27374)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27374
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Differential Revision: D17809770
Pulled By: ezyang
fbshipit-source-id: 75bd97426494a7bbbf08f9bce7563d35871443d8
