[caffe2][dnnlowp] Remove openmp usage in quantize dnnlowp op
Summary: It creates cpu overload issues when openmp gets enabled and OMP_NUM_THREADS=1 is not set.
Test Plan: buck test //caffe2/caffe2/quantization/server:quantize_dnnlowp_op_test
Reviewed By: jspark1105
Differential Revision: D24437305
fbshipit-source-id: 426209fc33ce0d4680c478f584716837ee62cb5e
