better error msg when seeing a unsupported builtin function (#21068)
Summary:
fixes https://github.com/pytorch/lockdown/issues/39.
Hopefully it doesn't break other tests....
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21068
Differential Revision: D15761895
Pulled By: ailzhang
fbshipit-source-id: 60cbb16cfc930b377d753b81e10b7edaea9a1281
