Add count_include_pad to average_pool_gradient_op (#15997)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15997
Add count_include_pad to average_pool_gradient_op
Reviewed By: houseroad
Differential Revision: D13648339
fbshipit-source-id: 205cb2acb32dc24a85256b628298b1a11f0ffa2c
