[SR] Implement prim::Loop (#69838)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69838
Implement `prim::Loop` with the new `StaticRuntimeBlockRunner` abstraction.
ghstack-source-id: 148186483
Test Plan: New unit tests: `buck test caffe2/benchmark/static_runtime/...`
Reviewed By: d1jang
Differential Revision: D33049595
fbshipit-source-id: 550de5167b46fccd65ff77d092785289b5e5d532
(cherry picked from commit 8baf1753af34f4c166b4680e42589517fd2e508d)
