Enumerate a set of anchors for each aspect ratio wrt an anchor.

def _ratio_enum(anchor, ratios):
        """
        Enumerate a set of anchors for each aspect ratio wrt an anchor.
        """
        w, h, x_ctr, y_ctr = AnchorGenerator._whctrs(anchor)
        size = w * h
        size_ratios = size / ratios
        ws = np.round(np.sqrt(size_ratios))
        hs = np.round(ws * ratios)
        anchors = AnchorGenerator._mkanchors(ws, hs, x_ctr, y_ctr)
        return anchors