Fix assertNotEqual error reporting (#39217)
Summary:
`msg` argument must be passed to `assertRaises`, because its exception is passed upstream (with custom error message) if `assertEquals` succeedes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39217
Differential Revision: D21786141
Pulled By: malfet
fbshipit-source-id: f8c3d4f30f474fe269e50252a06eade76d575a68
