Enumerate a set of anchors for each scale wrt an anchor.

def _scale_enum(anchor, scales):
        """
        Enumerate a set of anchors for each scale wrt an anchor.
        """
        w, h, x_ctr, y_ctr = AnchorGenerator._whctrs(anchor)
        ws = w * scales
        hs = h * scales
        anchors = AnchorGenerator._mkanchors(ws, hs, x_ctr, y_ctr)
        return anchors