Fix interval midpoint calculation (#46666)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46666
Interval midpoint calculations can overflow (integers) this diff fixes such an instance.
Test Plan: Standard test rig
Reviewed By: xw285cornell
Differential Revision: D23997893
fbshipit-source-id: 788c1181031e0b71d3efb6f7090fbd4ba2aa3f86
