Workaround unvectorized mean implementation (#16618)
Summary:
Workaround for https://github.com/pytorch/pytorch/issues/16617
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16618
Differential Revision: D13904276
Pulled By: jamesr66a
fbshipit-source-id: f8b5ea4c5f12dbc405123c9080c55b342c95bcd1
