Generate anchor (reference) windows by enumerating aspect ratios X
        scales wrt a reference (0, 0, 15, 15) window.

def _generate_base_anchors(base_size, scales, ratios):
        """
        Generate anchor (reference) windows by enumerating aspect ratios X
        scales wrt a reference (0, 0, 15, 15) window.
        """
        base_anchor = np.array([1, 1, base_size, base_size]) - 1
        ratio_anchors = AnchorGenerator._ratio_enum(base_anchor, ratios)
        anchors = np.vstack([AnchorGenerator._scale_enum(ratio_anchors[i, :], scales)
                             for i in range(ratio_anchors.shape[0])])
        return anchors