[PyTorch][easy] Don't copy string in TensorType::repr_str unnecessarily (#66699)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66699
std::string::operator+ will copy the string an extra time even if the argument is `""`. See https://godbolt.org/z/3sM5h1qTo
ghstack-source-id: 140743822
Test Plan: CI
Reviewed By: suo
Differential Revision: D31693522
fbshipit-source-id: 6a8033c90366904b9aff44214b600cfb255a0809
