`nll_loss_forward`: port to structured kernel (#61443)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61443
For more information, see #55070.
This PR also adds a new type, `OptionalTensorRef` as a replacement for `c10::optional<Tensor>&` in order to avoid the reference count manipulations that are inevitable with the latter. I have confirmed using Godbolt/Compiler Explorer that this class does indeed avoid manipulating the reference count of the `intrusive_ptr` inside the `Tensor` it refers to:
1. [P429709479](https://www.internalfb.com/phabricator/paste/view/P429709479) - Given a `const Tensor&` in scope, an `OptionalTensorRef` can be constructed without bumping refcount.
2. [P429709883](https://www.internalfb.com/phabricator/paste/view/P429709883) - Given an `OptionalTensorRef`, a `const Tensor&` can be produced without bumping refcount.
3. [P429710335](https://www.internalfb.com/phabricator/paste/view/P429710335) - When `OptionalTensorRef` is destructed, the refcount should not be decremented.
4. [P429769525](https://www.internalfb.com/phabricator/paste/view/P429769525) - `OptionalTensorRef` can be assigned without refcount manipulation.
5. [P429769882](https://www.internalfb.com/phabricator/paste/view/P429769882) - `OptionalTensorRef` can be move assigned without refcount manipulation.
Test Plan: Imported from OSS
Reviewed By: jamesr66a
Differential Revision: D29780666
Pulled By: SplitInfinity
fbshipit-source-id: 7af157215300e9254d635433cbd583f7329fe064
