Deprecate torch.lu
**BC-breaking note**:
This PR deprecates `torch.lu` in favor of `torch.linalg.lu_factor`.
A upgrade guide is added to the documentation for `torch.lu`.
Note this PR DOES NOT remove `torch.lu`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73804
Approved by: https://github.com/IvanYashchuk, https://github.com/mruberry
