Add missing comment from #22103 (#22984)
Summary:
One important comment is missing from https://github.com/pytorch/pytorch/issues/22103 (not sure what happened).
This commit makes it up.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22984
Differential Revision: D16347044
Pulled By: ezyang
fbshipit-source-id: 0903909a5fb6740b43195136f1a23c28cfb2a02f
