[SR] Implement prim::If (#69837)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69837
Implement `prim::If` with the new `StaticRuntimeBlockRunner` abstraction.
ghstack-source-id: 148186475
Test Plan:
New unit tests: `buck test caffe2/benchmarks/static_runtime/...`
Accuracy test at top of stack
Reviewed By: d1jang
Differential Revision: D33045908
fbshipit-source-id: 281fb4a73528249fa60f65ac26f8ae6737771f55
(cherry picked from commit de3b12dc0871e8ca09891c257e1dfd7cd352aa7c)
