Undo norm optimizations and add more documentation for parallel.h (#15885)
Summary:
See https://github.com/pytorch/pytorch/issues/15602
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15885
Differential Revision: D13614841
Pulled By: cpuhrsch
fbshipit-source-id: 5d3e45f499d36ac287dbbc2e45798aa51eb5bfdf
