Add instruction on how to nest nn::Sequential (#23939)
Summary:
yf225 I mentioned the MNASNet implementaiton in the message.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23939
Differential Revision: D16716619
Pulled By: yf225
fbshipit-source-id: a92e4e7a588decce4c5a515370238eb284ae6118
