[caffe2/nomnigraph] handle when PATH env is not defined (#39373)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39373
Line 114 is the only actual change. Other changes are just formatting.
Test Plan: CI
Reviewed By: zrphercule
Differential Revision: D21830893
fbshipit-source-id: 83e49b1b3c48f6bc6de3c48ccce60c84aa49339b
