[PyTorch][Static Runtime] Combine ProcessedNode::{native_,}fn_ (#65414)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65414
Saves 24 bytes (`sizeof(std::function) - 8`) per ProcessedNode.
ghstack-source-id: 139999909
Test Plan: CI
Reviewed By: hlu1
Differential Revision: D31085561
fbshipit-source-id: 70734b8319e805736ba41aedaaf7fa3d463400c9
