* .guix-channel: Add dependency on guix-science-nonfree.
* non-free/cuda.scm: Remove everything except the 'cuda' variable.
Re-export things from (guix-science-nonfree packages cuda).
* tainted/machine-learning.scm: Likewise, and mark "python-pytorch-cuda"
as deprecated.
* README.md: Mention guix-science-nonfree.