Add global pooling specialization and also update MaxPooling on GPU (#15824)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15824
Add global pooling specialization and also update MaxPooling on GPU
Reviewed By: houseroad
Differential Revision: D13596340
fbshipit-source-id: c8a42aa69ee92c383c9f19d3ed57b77cb3e5bd28
