Checkpointable: Make Model Checkpointable-compatible

Has Models track Checkpointable dependencies with __setattr__.

Switches subclassed Models to creating ResourceVariables by default, which
removes one source of eager/graph differences. tfe.Network was doing this by
default.

This is necessary for eager/graph agnostic code since tapes currently only work
with ResourceVariables. It's not quite trivial to fix that, and
ResourceVariables by default in more places is a Good Thing anyway. (Not that we
shouldn't also fix the tape code.)

PiperOrigin-RevId: 187544850