Deprecate torch.lstsq (#57743)
Summary:
**BC-breaking note:**
This PR deprecates torch.lstsq; it adds an upgrade guide for how to use torch.linalg.lstsq instead.
It DOES NOT remove torch.lstsq, but warns once when it's called
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57743
Reviewed By: bdhirsh
Differential Revision: D28318196
Pulled By: mruberry
fbshipit-source-id: 0d6df29648a91a44c7d0ac58062c1099fcb61fb8
