Turn on Travis builds for ghstack PRs. (#18193)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18193
ghimport-source-id: 540859cf0b238a9832f45b3f4c2351e3343fc1a2
Stack from [ghstack](https://github.com/ezyang/ghstack):
* **#18193 Turn on Travis builds for ghstack PRs.**
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Differential Revision: D14529945
fbshipit-source-id: 4476e996e311a04f2a997ca9b7c4cf2157dd6286
