message: Add tf.contrib.checkpoint.CheckpointableBase for isinstance checks.
  (Also planning to use this in Sonnet)
diff: @@ -22,6 +22,7 @@ Visualization and inspection:
  Managing dependencies:
  @@capture_dependencies
  @@Checkpointable
  +@@CheckpointableBase
  @@CheckpointableObjectGraph
  @@NoDependency
  @@split_dependency
  @@ -41,6 +42,7 @@ from tensorflow.contrib.checkpoint.python.split_dependency import split_dependen
  from tensorflow.contrib.checkpoint.python.visualize import dot_graph_from_checkpoint
  from tensorflow.core.protobuf.checkpointable_object_graph_pb2 import CheckpointableObjectGraph
  from tensorflow.python.training.checkpointable.base import Checkpointable
  +from tensorflow.python.training.checkpointable.base import CheckpointableBase
  from tensorflow.python.training.checkpointable.base import NoDependency
  from tensorflow.python.training.checkpointable.data_structures import List
  from tensorflow.python.training.checkpointable.data_structures import Mapping
  @@ -51,4 +53,3 @@ from tensorflow.python.training.checkpointable.util import object_metadata
  from tensorflow.python.util.all_util import remove_undocumented
  remove_undocumented(module_name=__name__)
  -
  