add BFloat16 support for AvgPool2d on CPU (#66927)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/66927
Test Plan: Imported from OSS
Reviewed By: mikaylagawarecki
Differential Revision: D33353198
Pulled By: VitalyFedyunin
fbshipit-source-id: 1aeaa4bb90ac99210b8f6051c09d6995d06ce3a1
