fix qat tests (#23124)
Summary:
missed instantiating observers in linear
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23124
ghstack-source-id: 86886705
Reviewed By: raghuramank100
Differential Revision: D16401066
fbshipit-source-id: f9f0f359caeca855c62192d13261a33eef57715a
