class_docstr: Map indexes from one layer to the other.
  
  Parameters
  ----------
  input_size : tuple
      the input size
  output_size : tuple
      the mapped input layer's output size
code: def __call__(self, index):
      flat_index = np.ravel_multi_index(index, self.__output_size)
      return np.unravel_index(flat_index, self.__input_size)
  
imports: import itertools
  import numpy
label_desc: Use image node_id 4 for calling the IndexMapper obj's underlying member method code with example usage: obj.__call__(index) and returns: np