Checkpointable: better handling of objects which aren't being restored

initialize_or_restore on a tf.train.Checkpoint status object will now initialize
any variables which aren't being restored, which is closer to the behavior when
executing eagerly (and makes it easier to use).

Fixes a bug where assert_consumed() would miss some Python objects which aren't
part of the object graph being restored. It will now (correctly/as documented)
complain about unmatched Python objects in the dependency graph.

PiperOrigin-RevId: 194315742