clean up import_source (#24282)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24282
This moves a test from Python to cpp, and in doing so lets us clean up a
bunch of otherwise unused code.
Test Plan: Imported from OSS
Differential Revision: D16800562
Pulled By: suo
fbshipit-source-id: ebc29bb81f4fb2538081fa309ead1739980f1093
