make conv_depthwise_dnnlowp_op_test faster (#15725)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15725
As title says.
Reviewed By: jianyuh
Differential Revision: D13579188
fbshipit-source-id: 382072c95929ccf9e189e2338e35b046c4a0650f
