Fix circular import error in torchgen (#81355)
This also formats `tools/pyi/gen_pyi.py` with `usort` to test the fix because that is how the bug was discovered. The
usort-formatted `gen_pyi.py` should work now without any issues
Fixes #81294
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81355
Approved by: https://github.com/ezyang
