[PyTorch] remove string_view::operator[] bounds check (#64670)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64670
Bounds checking is not required for `std::string_view`, and the checking hoses performance for the following performance prototype diff.
ghstack-source-id: 138037531
Test Plan: CI
Reviewed By: ezyang, bhosmer
Differential Revision: D30747515
fbshipit-source-id: 1f4374415a82dfdccce76ea2c6885c13cb93d369
