'Access list of groundtruth tensors.
Args:
field: a string key, options are
fields.BoxListFields.{boxes,classes,masks,keypoints}
Returns:
a list of tensors holding groundtruth information (see also
provide_groundtruth function below), with one entry for each image in the
batch.
Raises:
RuntimeError: if the field has not been provided via provide_groundtruth.'

def groundtruth_lists(self, field):

    if (field not in self._groundtruth_lists):
        raise RuntimeError('Groundtruth    tensor    %s    has    not    been    provided', field)
    return self._groundtruth_lists[field]
