Update docs to include variable annotation example (#16324)
Summary:
Relates to this issue https://github.com/pytorch/pytorch/issues/16288
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16324
Reviewed By: ezyang
Differential Revision: D13805412
Pulled By: suo
fbshipit-source-id: 8b80f988262da2c717452a71142327bbc23d1b8f
