update tests to run back-compat check using new binary (#41949)
Summary:
instead exporting schemas using the current binary being tested, install nightly and export its schemas to use in a back-compat test run by the current binary being tested.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41949
Reviewed By: houseroad
Differential Revision: D22731054
Pulled By: bradleyhd
fbshipit-source-id: 68a7e7637b9be2604c0ffcde2a40dd208057ba72
