add a github workflow to run CI on Windows

[NickleDave]

I think this will work now that we're not using poetry? Need to test on a Windows machine whether tests run -- not sure if installing pytorch with pure pip works, may need to do a per-platform specification in the dependencies in pyproject.toml -- I think you can do this with environment markers? https://www.python.org/dev/peps/pep-0508/#environment-markers