Add an option to stop the build process once cmake terminates. (#21034)
Summary:
Add an option to setup.py to stop the build process once cmake terminates. This leaves users a chance to fine adjust build options. Also update README accordingly.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21034
Differential Revision: D15530096
Pulled By: soumith
fbshipit-source-id: 71ac6ff8483c3ee77c38d88f0d059db53a7d3901
