Conversion class name string => integer.

def str2int(self, str_value):
    """Conversion class name string => integer."""
    str_value = tf.compat.as_text(str_value)
    if self._str2int:
      return self._str2int[str_value]

    # No names provided, try to integerize
    failed_parse = False
    try:
      int_value = int(str_value)
    except ValueError:
      failed_parse = True
    if failed_parse or not 0 <= int_value < self._num_classes:
      raise ValueError("Invalid string class label %s" % str_value)
    return int_value