move build_variables.bzl and ufunc_defs.bzl from pytorch-root/tools/ to the root
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78542
This makes importing easier in different build systems that have
different absolute names for the pytorch-root.
Differential Revision: [D36782582](https://our.internmc.facebook.com/intern/diff/D36782582/)
**NOTE FOR REVIEWERS**: This PR has internal Facebook specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D36782582/)!
Approved by: https://github.com/malfet
Author
Michael Andreas Dagitses
