Checkpointable: compatibility mode with name-based saving

Allows loading a name-based checkpoint using the object-based API. When
graph building it's quite seamless. There's no restore-on-create for
eager, so it would require program changes to do much useful there (i.e.
is not seamless).

Adds several tests for checkpoint compatibility (name->object in eager/graph,
and eager->graph/graph->eager for object-based saving)

PiperOrigin-RevId: 186844431