Builds a static tf.train.Saver from a checkpointable object graph

Moves around some SaveableObjects to support a freeze method for python state saveables, and makes sure that the object graph proto is included in the frozen Saver.

This should be useful for embedding in SavedModels, where variables can be updated and the resulting checkpoints (saved from the SaverDef in the SavedModel) will still support Keras-style object-based restoration into Python programs (with better eager support and less fragile variable matching). This is also a step toward Estimators saving object-based checkpoints.

PiperOrigin-RevId: 212017296