If ninja is being used, force build_ext to run. (#40881)
As ninja has accurate dependency tracking, if there is nothing to do,
then we will very quickly noop. But this is important for correctness:
if a change was made to a header that is not listed explicitly in
the distutils Extension, then distutils will come to the wrong
conclusion about whether or not recompilation is needed (but Ninja
will work it out.)
This caused https://github.com/pytorch/vision/issues/2367
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
ghstack-source-id: 6409595c8ac091f3863f305c123266b9d3a167ad
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40837
