Drop some unused variables (#70879)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70879
Sandcastle from layer_norm_kernel.cu
Test Plan: Sandcastle
Reviewed By: malfet
Differential Revision: D33439040
fbshipit-source-id: e7d0e37ab25d62c63f675da3b6eff670fd93b26a
