Relax check on outputs (#15458)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15458
many nets in the wild seem to have outputs that are never produced by the net.
Reviewed By: ZolotukhinM
Differential Revision: D13534185
fbshipit-source-id: 2b23b39c28404c53f68868f3bf6df53c5fea9eab
