🚀 Feature

This is a follow up on #50448, some more python tests are still using unittest.main()

Not sure if they are intended since they are not using common_utils but here is the list.
test/custom_backend/*
test/custom_operator/*
test/mobile/*
test/onnx/*
modules/detectron/*
scripts/release_notes

in addition there are many more tests in /caffe2/ folder which I assume we will not address at the moment.

cc @mruberry @VitalyFedyunin @walterddr