'Constructor.
Args:
is_training: Indicates whether the BoxPredictor is in training mode.
num_classes: number of classes.  Note that num_classes *does not*
include the background category, so if groundtruth labels take values
in {0, 1, .., K-1}, num_classes=K (and not K+1, even though the
assigned classification targets can range from {0,... K}).'

def __init__(self, is_training, num_classes):

    self._is_training = is_training
    self._num_classes = num_classes
