C++ API parity: AdaptiveAvgPool2d
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/26818
Test Plan: Imported from OSS
Differential Revision: D17627822
Pulled By: pbelevich
fbshipit-source-id: 0e1dea1c3ff2650dbc7902ce704ac6b47588d0bb
