use ufunc_defs.bzl in BUILD.bazel
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77747
This allows us to remove some duplication in aten.bzl.
Differential Revision: [D36481025](https://our.internmc.facebook.com/intern/diff/D36481025/)
**NOTE FOR REVIEWERS**: This PR has internal Facebook specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D36481025/)!
Approved by: https://github.com/seemethere
Author
Michael Andreas Dagitses
