Fix tracing docs and add more comprehensive examples (#22082)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/21857
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22082
Differential Revision: D15968306
Pulled By: Krovatkin
fbshipit-source-id: a76e500b0b7192bd814931ec48bbe9c37b8b92e0
