Clear cmake cache when --cmake (#16426)
Summary:
Also, because sometimes we have `CMakeCache.txt` but cmake errored out so I'm adding the existence of `'build.ninja'` as another criterion of rerunning cmake.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16426
Differential Revision: D13843801
Pulled By: ezyang
fbshipit-source-id: ea1efb201062f23b7608f8d061997d8a8e293445
