Add an API to register objects to Auto classes
# What does this PR do?

This PR adds an API to register custom to the auto classes.

Fixes #13522
Thanks a lot for working on this! I think this would greatly benefit from having an entry/tutorial in the `Auto Classes` section:

![image](https://user-images.githubusercontent.com/30755778/137330917-4ccf2167-02db-4378-9234-9cab4f54df52.png)


WDYT?<|||||>Great idea!