[c2] fix compute_norm test (#38529)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38529
(Note: this ignores all push blocking failures!)
Test Plan: buck test mode/opt //caffe2/caffe2/python/modeling:compute_norm_for_blobs_test
Reviewed By: olittle
Differential Revision: D21588603
fbshipit-source-id: bdb0ae455e85a934cb5e369fbb0078f2ff842814
