avg_pool2d_backward: Port to structured (#59082)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/59082
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D28802621
Pulled By: ezyang
fbshipit-source-id: 15b8ba562eee132ef8390a7de520bdd8e15d0f86
