[PyTorch] Reduce move overhead in inferExpandGeometry (#56032)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56032
Profiling & assembly inspection showed that we weren't
getting NRVO with `inferExpandGeometry_dimvector` returning
`std::tuple`. I added a custom type with constructors so that, as the
comment says, we could be sure to get NRVO.
ghstack-source-id: 127409717
Test Plan:
Inspected new assembly, no more move construction (which is
a copy for on-stack DimVectors!) upon returning
Reviewed By: ezyang
Differential Revision: D27768312
fbshipit-source-id: d1d53a36508be92585802e1467d8a42d1ae05d80
