Separate reduce functions from math (#16929)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16929
Separate CPU reduce functions from math
i-am-not-moving-c2-to-c10
Reviewed By: houseroad
Differential Revision: D13999469
fbshipit-source-id: bd628b15a6e3c1f04cc62aefffb0110690e1c0d1
