Implement avg_pool2d kernel for channels_last (#35855)
Summary:
Implement avg_pool2d for channels_last. This will close https://github.com/pytorch/pytorch/issues/34996.
Performance compared with **avg_pool2d** contiguous can be found at https://github.com/xwang233/code-snippet/blob/ed6617c6bc48dac5757d9a1ca6f5db5a68e5d01b/avg-pool2d-channels-last/avg-pool2d-naive.ipynb
cc csarofeen ptrblck
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35855
Differential Revision: D21187360
Pulled By: VitalyFedyunin
fbshipit-source-id: b654b56168bc3982be306b634c7ed2f92018a9e5
