Wrapping namespace Reduction in namespace at (#26606) (#27422)
Summary:
1) Wrapped namespace `Reduction` in namespace `at`
2) Prefixed `at::` wherever `Reduction::` is used
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27422
Differential Revision: D17913759
Pulled By: yf225
fbshipit-source-id: 8f00ca01cad2e7f673d316b128abf59c026e216c
