[Aten] Init container iterators to an unsigned type (#34159)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34159
This fixes `comparison of integers of different sign` warnings
Test Plan: CI
Reviewed By: EscapeZero
Differential Revision: D20232085
fbshipit-source-id: 8f325be54395be54c704335cb7edf2ec7ef75e75
