Fix error checking of CUDA multi_margin_loss. (#30825)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30825
It didn't verify in the 1-d case that the targets were size 1..
Test Plan: Imported from OSS
Differential Revision: D18833659
Pulled By: gchanan
fbshipit-source-id: 9b0276e7b0423fdaf2ba7cfa34bde541558c61f9
