[static-runtime] optimize empty if blocks at runtime (#74987)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74987
Add specializations to `prim::If` operator at runtime to save resources when some of subblocks are empty
Test Plan:
`buck build //caffe2:torch-cpp-cpu`
`buck test //caffe2/benchmarks/static_runtime/...`
Add unit test:
`buck test //caffe2/benchmarks/static_runtime:static_runtime_cpptest -- StaticRuntime.EmptyIfBlock`
Reviewed By: mikeiovine
Differential Revision: D35262952
fbshipit-source-id: 324f88471f33f035f4d8a9b212716530d8e59df2
(cherry picked from commit 2db1b1a6833b1376fa376f54791effc8e12fb77f)
