message: Simplify TrackableResource example usage
  _initialize() and _destroy_resource() don't need to return anything.
diff: @@ -317,10 +317,10 @@ class TrackableResource(CapturableResource):
  ... def _create_resource(self):
  ... return tf.raw_ops.VarHandleOp(dtype=tf.float32, shape=[2])
  ... def _initialize(self):
  - ... return tf.raw_ops.AssignVariableOp(
  + ... tf.raw_ops.AssignVariableOp(
  ... resource=self.resource_handle, value=tf.ones([2]))
  ... def _destroy_resource(self):
  - ... return tf.raw_ops.DestroyResourceOp(resource=self.resource_handle)
  + ... tf.raw_ops.DestroyResourceOp(resource=self.resource_handle)
  >>> class DemoModule(tf.Module):
  ... def __init__(self):
  ... self.resource = DemoResource()
  