Minor bug fix in dnnlowp (#15841)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15841
Fix the bugs in dnnlowp to support int8/int16 quantization for sparsenn.
Reviewed By: jspark1105
Differential Revision: D13600878
fbshipit-source-id: 27f06d7c54a663208320c8f211714220a9b49540
