Significantly simplify the spawning of pytorch libs building process. (#21105)
Summary:
Instead of attempting to hardcode calls to "ninja" or "make", we should always let cmake do it. This better integrates build configurations (DEBUG or REL_WITH_DEB_INFO) and better handles the case in which the native build tool is not in PATH (cmake has some capacity to find them and has options for users to specify their locations).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21105
Differential Revision: D15602883
Pulled By: soumith
fbshipit-source-id: 32ac46d438af00e791defde6ae5ac21c437d0bb0
