Remove boxes which contains at least one side smaller than min_size.

    Args:
        boxes (Tensor[N, 4]): boxes in ``(x1, y1, x2, y2)`` format
            with ``0 <= x1 < x2`` and ``0 <= y1 < y2``.
        min_size (float): minimum size

    Returns:
        Tensor[K]: indices of the boxes that have both sides
        larger than min_size
    Nr