Fix JIT tests when run locally in fbcode (#45776)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45776
Splitting out backend and custom class registration into their own library is
not currently implemented in fbcode, so detect that we are running tests in
fbcode and disable those tests.
Test Plan: buck test mode/no-gpu mode/dev caffe2/test:jit
Reviewed By: smessmer
Differential Revision: D24085871
fbshipit-source-id: 1fcc0547880bc4be59428e2810b6a7f6e50ef798
