[op-bench] modify import path of configs (#46679)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46679
Current way of import configs will have runtime error when a single benchmark is launched directly with buck(e.g. `/buck-out/gen/caffe2/benchmarks/operator_benchmark/pt/conv_test.par`). The diff fixed that issue.
ghstack-source-id: 114857978
Test Plan: waitforsandcastle
Reviewed By: vkuzo
Differential Revision: D24459631
fbshipit-source-id: 29df17e66962a8604dbb7b8b9106713c3c19bed5
