Initializes batches by reading from the datastore.

def init_from_datastore(self):
    """Initializes batches by reading from the datastore."""
    self._data = {}
    for entity in self._datastore_client.query_fetch(
        kind=self._entity_kind_batches):
      batch_id = entity.key.flat_path[-1]
      self._data[batch_id] = dict(entity)
      self._data[batch_id]['images'] = {}
    for entity in self._datastore_client.query_fetch(
        kind=self._entity_kind_images):
      batch_id = entity.key.flat_path[-3]
      image_id = entity.key.flat_path[-1]
      self._data[batch_id]['images'][image_id] = dict(entity)