Fix unused variable warning in layer_norm_kernel.cu (#71587)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/71587
Test Plan: Sandcastle
Reviewed By: meyering
Differential Revision: D33692615
fbshipit-source-id: 84d249eb967fb97191aaf59c9a96cac9f8cae498
(cherry picked from commit ca36d04c3166dbeac73ef88ecc215c0b426169dc)
