Adds batch with give ID and list of properties.

def add_batch(self, batch_id, batch_properties=None):
    """Adds batch with give ID and list of properties."""
    if batch_properties is None:
      batch_properties = {}
    if not isinstance(batch_properties, dict):
      raise ValueError('batch_properties has to be dict, however it was: '
                       + str(type(batch_properties)))
    self._data[batch_id] = batch_properties.copy()
    self._data[batch_id]['images'] = {}