[caffe2] Don't evaluate message in CAFFE_ENFORCE_THAT unless the check fails (#106145)
D26829714 improved CAFFE_ENFORCE_THAT, but made us eagerly evaluate the message, which has costs.
Differential Revision: [D47809432](https://our.internmc.facebook.com/intern/diff/D47809432/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106145
Approved by: https://github.com/davidberard98
