Fix ImportError in the OSS land. (#43912)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43912
Fixed the ImportError: cannot import name 'compute_ulp_error' from 'caffe2.python.oss.fakelowp.test_utils'
Test Plan: test_op_nnpi_fp16.py
Reviewed By: hyuen
Differential Revision: D23435218
fbshipit-source-id: be0b240ee62090d06fdc8efac85fb1c32803da0d
