fix input schema check for spatialbn
Summary:
we were restricting it to 3, but in training we set up to 5, even that
in practice we just need 3 since we don't recompute mean/var
Test Plan: contrib tests for fakelowp
Reviewed By: hl475
Differential Revision: D21905490
fbshipit-source-id: 48f61c7ba7d95f19d55d2f65514a517c1514ae88
