Port avg_pool3d() to ATen (#21732)
Summary:
This will need a conflict resolution once avg_pool2d() has been merged.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21732
Differential Revision: D15824923
Pulled By: ezyang
fbshipit-source-id: 83341e0209b660aecf788272079d8135d78b6ff1
