message: Add a device field to the object graph Variable proto.
  This is in preparation for supporting saving variable devices by using
  the experimental SAVE_VARIABLE_DEVICES SaveOptions flag.
diff: @@ -140,6 +140,7 @@ message SavedVariable {
  VariableSynchronization synchronization = 4;
  VariableAggregation aggregation = 5;
  string name = 6;
  + string device = 7;
  }
  // Represents `FunctionSpec` used in `Function`. This represents a
  