Checkpointable: Don't run ops automatically when graph building.

This is a prerequisite to moving toward a Saver-like model when graph building. We no longer mess with initializers (when graph building; eager needs it), and restore ops just get queued up and returned.

Since initializers are left alone when graph building, there is a new special case for slot variables which needs to be handled. This is the third(!) queue for deferred slot restorations ((1) variable -> slot, (2) optimizer -> slot, (3) (optimizer, variable) -> slot), and should be the last one I need (it's a hypergraph with 3-tuple edges).

The plan after this is to switch over to tf.train.Saver's existing restore op creation infrastructure, which will handle any SaveableObjects. There will also be a few CLs for making graph usage prettier, and eventually allowing eager/graph agnostic save/restore.

PiperOrigin-RevId: 186059387