Consolidated duplicate tests for model cloning and added Keras test decorators where appropriate.

Updated models_tests to run in v1/v2. This required several other changes:
* Added a testing utility to get v2 optimizers
* Ensured that subclassed models cleared key compilation attrs upon clone_reset (_is_compiled; optimizer)
* Modified train/eval loops to check self._is_compiled rather than the existence of other attrs created during compiling, as these other attrs would not be reset during cloning.

PiperOrigin-RevId: 227938218