[torch] Fix sign-compare warning in `torch::utils::rnn:pack_sequence` (#34185)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34185
ArrayRef<T>::size() is size_t
Test Plan: CI
Reviewed By: EscapeZero
Differential Revision: D20241552
fbshipit-source-id: 73cd062db810ebc5a4e34e094dfe6c7e6571ef2d
