Nicer promotion error message when pr. (#27941)
Summary:
Instead of an abstruse "unsupported scalarType", we print more.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27941
Differential Revision: D17933972
Pulled By: ezyang
fbshipit-source-id: 51e0e1c11e530606612482e24ff28898323e54fc
