examples/research_projects/lxmert/modeling_frcnn.py/convert_boxes_to_pooler_format
def convert_boxes_to_pooler_format(box_lists: List[torch.Tensor]):
    pooler_fmt_boxes = torch.cat(
        [_fmt_box_list(box_list, i) for i, box_list in enumerate(box_lists)],
        dim=0,
    )
    return pooler_fmt_boxes