Minor OptionalTensorRef updates (#63611)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63611
A few minor updates to `OptionalTensorRef`:
1. use `Tensor`'s `unsafe_borrow_t` constructor which avoids an unnecesary `nullptr` check.
2. copy constructor cannot defer to the `const Tensor&` constructor because it checks the tensor is
defined, and so would fail for disengaged optionals.
3. use copy-swap idiom to avoid issues with self-assignment. `x = x` should be a no-op, but the old
version would clear `x`.
4. Add pointer-like access for consistency with `optional` and `MaybeOwned`
Test Plan: Imported from OSS
Reviewed By: bdhirsh
Differential Revision: D30484704
Pulled By: ezyang
fbshipit-source-id: 738f4bd22359eaecd0a519a04e89a4b44d92da5b
