'Decode.
Args:
rel_codes: a tensor representing N anchor-encoded boxes.
anchors: BoxList of anchors.  We assume that anchors has an associated
stddev field.
Returns:
boxes: BoxList holding N bounding boxes
Raises:
ValueError: if the anchors BoxList does not have a stddev field'

def _decode(self, rel_codes, anchors):

    if (not anchors.has_field('stddev')):
        raise ValueError('anchors    must    have    a    stddev    field')
    means = anchors.get()
    stddevs = anchors.get_field('stddev')
    box_corners = ((rel_codes * stddevs) + means)
    return box_list.BoxList(box_corners)
