[FX] Add docs for shape propagation (#54554)
Summary:
Fixes #{i54538}
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54554
Reviewed By: nikithamalgifb
Differential Revision: D27281263
Pulled By: jamesr66a
fbshipit-source-id: 2fd3914f0e24be0b6a18ad7715f3336dcf7949ba
