Add test for stacked "for_subclass_implementers" in class __mro__.
Add test for stacked "for_subclass_implementers" in class __mro__.

This test asserts that "for_subclass_implementers" short-circuits the __mro__ inspection.
A method labeled "for_subclass_implementers" will be documented in it's defining class, regardless of inheritable decorations farther up the class hierarchy.

PiperOrigin-RevId: 209802006

tensorflow/docs-l10n,tensorflow/docs,tensorflow/docs,tensorflow/docs