The `.weights` attribute and `TrainableModel` no longer exist; handling both model types now makes the example much more complex and prohibits usefulness.

Additionally:
- Fix backticks in pydoc.
- Rename "fields" to "attributes" in pydoc.
- Replace federated_value with more preferred federated_eval.
- Replace collections.namedtuple usage with attr.s classes.
- Improve determinism for unittests.
PiperOrigin-RevId: 395949705