code: def get_possible_initializer_keys(cls, num_layers):
      
      keys = [cls.WT, cls.WH]
      for layer_index in xrange(num_layers):
        layer_str = str(layer_index)
        keys += [
            cls.WT + layer_str,
            cls.BT + layer_str,
            cls.WH + layer_str,
            cls.BH + layer_str]
      return set(keys)
docs: Returns the keys the dictionary of variable initializers may contain.
  
      The set of all possible initializer keys are:
        wt: weight for input -> T gate
        wh: weight for input -> H gate
        wtL: weight for prev state -> T gate for layer L (indexed from 0)
        whL: weight for prev state -> H gate for layer L (indexed from 0)
        btL: bias for prev state -> T gate for layer L (indexed from 0)
        bhL: bias for prev state -> H gate for layer L (indexed from 0)
  
      Args:
        num_layers: (int) Number of highway layers.
      Returns:
        Set with strings corresponding to the strings that may be passed to the
          constructor.