MultiMarginCriterion: fix scalar_check in the case where reduction == None. (#30826)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30826
Previously the scalar_check for the reduction None case was:
input.dim() <= 1, but it should be target based, i.e.:
target.dim() == 0. This follows from the "correct cases", i.e.
(N, C) X (N,) -> (N,)
(C,) X () -> ()
Test Plan: Imported from OSS
Differential Revision: D18833660
Pulled By: gchanan
fbshipit-source-id: 26338b842a8311718c4b89da3e2f1b726d5409b8
