[SR] Graph pass to create owned refs of special IValues (#69835)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69835
`StaticRuntimeBlockRunner` moves its outputs to the return value at the end of `run_impl`. However, there's a corner case where this can cause problems. If we return a constant, then the only reference in the `constants_` array can be destroyed by this move. We could add special logic to handle this in `run_impl`. But since this is a relatively rare corner case, it's simpler to just add an op that does nothing but create an owned reference to its input. This owned reference can be safely moved out of `StaticRuntimeBlockRunner`.
Note that this also applies to returned values in sub-blocks that are from outer scopes.
ghstack-source-id: 148186452
Test Plan:
`buck test caffe2/benchmarks/static_runtime/...`
Added a new unit test with a graph that simply returns a constant.
Tests with sub-blocks at top of stack.
Reviewed By: d1jang
Differential Revision: D33047519
fbshipit-source-id: 22b6058f0d1da8a6d1d61a6f2866bc518bff482b
(cherry picked from commit a8f89a12ee726aa7d7e546dee25d696eef868ce7)
