benchmark

b45277a4 - cache Source.guard_source (#168386)

cache Source.guard_source (#168386) Summary: Reduces guard build time for a bit. Noticeable when guards get really large (e.g. DEPTH = 2000 in the repro). On the guard build benchmark, time went from 57.25s -> 50.18s X-link: https://github.com/pytorch/pytorch/pull/168386 Approved by: https://github.com/anijain2305 ghstack dependencies: #168131, #168203 Reviewed By: huydhn Differential Revision: D88341345 fbshipit-source-id: 78c358fa9919eba3309ec0660a228b1286692777