Fix type hint for `torch.Tensor.grad_fn` (#96804)
Fix type hint for `torch.Tensor.grad_fn`, which can be a `torch.autograd.graph.Node` or `None`.
This is a regression in `torch` 2.0. It makes `mypy` failure in downstream projects.
Ref:
- https://github.com/pytorch/pytorch/issues/94937#issuecomment-1469344993
- metaopt/torchopt#149
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96804
Approved by: https://github.com/Skylion007
