[PyTorch] IValue::toTensor can now return const Tensor& (#48868)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48868
Building on the previous diff, we can make `toTensor()` return a
`const Tensor&`, which should make it easier to avoid reference
counting.
ghstack-source-id: 119327372
Test Plan: internal benchmarks.
Reviewed By: bwasti
Differential Revision: D25325379
fbshipit-source-id: ca699632901691bcee432f595f75b0a4416d55dd
