[SR] Add BlockRunner and handle sub-blocks (#69834)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69834
* Modify the `StaticModule` constructor to handle index initialization for sub-blocks.
* Add a new class `StaticRuntimeBlockRunner`. This class is almost exactly like what we've been calling `StaticRuntime` up to this point, except that it does not own a `values_` array. All `StaticRuntimeBlockRunners` hold an unowned reference to a `values_` array owned by `StaticRuntime`. This is a useful abstraction for implementing control flow - it gives us a way for sub-blocks to look up values from surrounding scopes!
ghstack-source-id: 148086245
Test Plan: `buck test caffe2/benchmarks/static_runtime/...`
Reviewed By: d1jang
Differential Revision: D33028039
fbshipit-source-id: 4f01417bad51a0cf09b1680a518308da647be1f6
(cherry picked from commit 3a9feffd929869120c717d35aa55aad8a382783d)
