[PyTorch] Introduce packed SizesAndStrides abstraction (#47507)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47507
This introduces a new SizesAndStrides class as a helper for
TensorImpl, in preparation for changing its representation.
ghstack-source-id: 119313559
Test Plan:
Added new automated tests as well.
Run framework overhead benchmarks. Results seem to be neutral-ish.
Reviewed By: ezyang
Differential Revision: D24762557
fbshipit-source-id: 6cc0ede52d0a126549fb51eecef92af41c3e1a98
