Fix implicit conversion (#46833)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46833
Implicit integer conversions are causing compiler warnings. Since in this case the logs make it pretty clear that the `unsigned` types won't overflow despite 64-bit inputs, we fix the issue by making the downconversion explicit.
Test Plan: Standard test rig.
Reviewed By: malfet
Differential Revision: D24481377
fbshipit-source-id: 4422538286d8ed2beb65065544016fd430394ff8
