[PyTorch] Save some space in ProcessedNode (#48861)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48861
`std::function` already has an empty state; no need to wrap
it in `c10::Optional`.
ghstack-source-id: 117891382
Reviewed By: hlu1
Differential Revision: D25296912
fbshipit-source-id: 8291bcf11735d49db17415b5de915591ee65f781
