Add scary comment in cpp_custom_type_hack.h (#48737)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/48737
Test Plan: Imported from OSS
Reviewed By: dzhulgakov
Differential Revision: D25280542
Pulled By: jamesr66a
fbshipit-source-id: 67c3b8c82def848ba3059dd6f6a23f9c5e329c0f
