[PyTorch] Use std::find in the JIT lexer (#64652)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64652
If nothing else, it is slightly clearer code.
ghstack-source-id: 137608456
Test Plan: CI
Reviewed By: dhruvbird
Differential Revision: D30739239
fbshipit-source-id: bc7917b59883ca4a33fc6916b4e422bad79cf04b
