[SR] Force sub-blocks to return at least one output (#69836)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69836
It is technically possible for the sub-blocks to return zero outputs. This is problematic for `StaticRuntimeBlockRunner`, because it assumes that at least one output is being returned.
Rather than slowing down SR with special logic for this corner case, we can simply force these sub-blocks to return `None`.
ghstack-source-id: 148186453
Test Plan: Sub-blocks with no return values tested at top of stack
Reviewed By: d1jang
Differential Revision: D33050420
fbshipit-source-id: 17d9e19fda6431aa9fd0b155131349bac42bc149
(cherry picked from commit c97fd07bf53e1e253a0e6c733db5ea7c86698fc9)
