fix nest openmp performance bug in thnn_conv2d (#52577)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/52577
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D27063800
Pulled By: VitalyFedyunin
fbshipit-source-id: 000e17b722b2b1d48e1012b3fa222729e26777fb
