[PyTorch] Avoid initializing storage for empty Optionals
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78947
We don't need to initialize for the non-constexpr case ever, or in the constexpr case after C++20.
Differential Revision: [D36519379](https://our.internmc.facebook.com/intern/diff/D36519379/)
Approved by: https://github.com/ezyang, https://github.com/malfet
