Add ops for BI (mean) (#68826)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/68826
Test Plan: Imported from OSS
Reviewed By: samdow
Differential Revision: D32732465
Pulled By: eellison
fbshipit-source-id: e8b185d89e5ecbe5c8e09d576c84a1f0a402a5e0
