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.

def get_possible_initializer_keys(cls, num_layers):
    """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.
    """
    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)