Use "important" node to toggle whether or not to build on PR (#21308)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21308
ghimport-source-id: 75fba872a658d8257a3f6ff9d9e33a320c6e523e
Differential Revision: D15621909
Pulled By: ezyang
fbshipit-source-id: 6d016d9ffdeb6414d70a1b48ed4766b5dc626353
