class_docstr: Represents a Node in a tree containing a layer.
  
  `LayerNode` is used to represent a tree of layers in a model. It contains
  config which describes the layer, and other input layers feeding into it.
  
  It is used as a generic class to represent both sets of layers which have
  been found in a model, and layers which should be replaced inside the model.
code: def __str__(self):
      return "{} <- [{}]".format(
          self.layer,
          ", ".join(
              [str(input_layer) for input_layer in self.input_layers]
          ),
      )
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import abc
  import collections
  import six
label_desc: Use image node_id 2 for calling the LayerNode obj's underlying member method code with example usage: obj.__str__() and returns: str