[PyTorch] Support MaybeOwned<IValue> (#68157)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68157
Does what it says on the tin. I don't have a use for `MaybeOwned<IValue>` itself right now, but following diffs will use `MaybeOwnedTraits<IValue>::{create,destroy}Borrow` and I thought it best to just provide the full thing.
ghstack-source-id: 143424915
Test Plan: Extended MaybeOwned tests to cover this.
Reviewed By: hlu1
Differential Revision: D32347393
fbshipit-source-id: 219658cb69b951d36dee80c2ae51387328224866
