Initializes data by reading it from the datastore.

def init_from_datastore(self):
    """Initializes data by reading it from the datastore."""
    self._data = {}
    client = self._datastore_client
    for entity in client.query_fetch(kind=KIND_CLASSIFICATION_BATCH):
      class_batch_id = entity.key.flat_path[-1]
      self.data[class_batch_id] = dict(entity)