Properly check that reduction strings are valid for l1_loss, smoothl1_loss, and mse_loss. (#43527)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/43527
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D23306786
Pulled By: gchanan
fbshipit-source-id: f3b7c9c02ae02813da116cb6b247a95727c47587
