[Pytorch Edge] Minor improve documentation in test_backend_with_compiler
Summary:
Went through all these files and the design doc to understand the to_backend api. Figured I could add some comments to these files to make the apis a little clearer for those that come after.
(Note: this ignores all push blocking failures!)
Test Plan: na
Reviewed By: raziel, larryliu0820
Differential Revision: D34221989
fbshipit-source-id: 699fcbd8714bfb6b58c6c0bf0e5fbc019d2ef6f8
(cherry picked from commit 0b3f5d73e8de216b4402aed2f2b80be8781f145b)
