Show the `ImageBBox` on `ax`.

def show(self, y:Image=None, ax:plt.Axes=None, figsize:tuple=(3,3), title:Optional[str]=None, hide_axis:bool=True,
        color:str='white', **kwargs):
        "Show the `ImageBBox` on `ax`."
        if ax is None: _,ax = plt.subplots(figsize=figsize)
        bboxes, lbls = self._compute_boxes()
        h,w = self.flow.size
        bboxes.add_(1).mul_(torch.tensor([h/2, w/2, h/2, w/2])).long()
        for i, bbox in enumerate(bboxes):
            if lbls is not None: text = str(lbls[i])
            else: text=None
            _draw_rect(ax, bb2hw(bbox), text=text, color=color)