fix missing constant in adaptive_avg_pool2d AD (#17180)
Summary:
Thanks ngimel for pointing this out!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17180
Differential Revision: D14113001
Pulled By: ailzhang
fbshipit-source-id: 78e7d7f2cda3889138e2bf26a54980c2cc665882
