Dictionary tracking for tf.keras.Model attribute assignment

Does not inherit from dict (and so won't pass isinstance checks). I've written a small tome about why in a comment on the class definition. This seems not to break anyone, but if it does we can add Mapping to the problematic isinstance checks (as I've done for TF's nest util and Session fetching); ideally custom mappings would be supported everywhere dicts are anyway.

PiperOrigin-RevId: 205898305