Port `mean` kernel to structured kernels. (#61643)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61643
Tracking issue: #55070
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D29783866
Pulled By: ezyang
fbshipit-source-id: dc95baf593096c03fb5f292ee6c36de3cc7f2b35
