def METHOD_NAME(self):
    """
    Returns:
        int: number of anchors at every pixel
            location, on that feature map.
            For example, if at every pixel we use anchors of 3 aspect
            ratios and 5 sizes, the number of anchors is 15.
            For FPN models, `num_anchors` on every feature map is the same.
    """
    return len(self.cell_anchors[0])