Add comment with UNBOXEDONLY explanation to codegen (#40117)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40117
ghstack-source-id: 106804731
Test Plan: just comments
Reviewed By: ezyang
Differential Revision: D22075103
fbshipit-source-id: 76677dc337196b71c50075f2845a1899451a705f
