'all_boxes is a list of length number-of-classes.
Each list element is a list of length number-of-images.
Each of those list elements is either an empty list []
or a numpy array of detection.
all_boxes[class][image] = [] or np.array of shape #dets x 5'

def evaluate_detections(self, all_boxes, output_dir=None):

    raise NotImplementedError
