Wrap workaround for cpp custom types a bit prettier and add an example (#18791)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18791
As a temporary demonstration on how to extend this hack further until custom C types are ready.
Reviewed By: jamesr66a
Differential Revision: D14742020
fbshipit-source-id: 0f2fd83ae56ab2abe16977a1829ed421e6abe74b
