class_docstr: Base layer class.
  
  Parameters
  ----------
  input_size : list
      size of the layer input
  output_size : list
      size of the layer output
  activation : str or None
      activation function name
  input_index_mapper : IndexMapper or None
      map indexes from this layer index to the input layer index size
docstr: Return the index mapper
docstr_tok: ["Return","the","index","mapper"]
code: def input_index_mapper(self):
      """Return the index mapper"""
      return self.__input_index_mapper
  
imports: import itertools
  import numpy
label_desc: Use image node_id 6 for calling the Layer obj's underlying member method code with example usage: obj.input_index_mapper() and returns: self