Use slimmer exception raising code when on mobile. (#20543)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20543
All of that code for concatenating strings together adds up. Just discard it all for mobile builds.
Reviewed By: ljk53
Differential Revision: D15353447
fbshipit-source-id: a82dd0b884335d662605aabf7dd3d09dfcc1478b
