Generates a matrix of anchor boxes in (x1, y1, x2, y2) format. Anchors
    are centered on stride / 2, have (approximate) sqrt areas of the specified
    sizes, and aspect ratios as given.

def generate_anchors(
    stride=16, sizes=(32, 64, 128, 256, 512), aspect_ratios=(0.5, 1, 2)
):
    """Generates a matrix of anchor boxes in (x1, y1, x2, y2) format. Anchors
    are centered on stride / 2, have (approximate) sqrt areas of the specified
    sizes, and aspect ratios as given.
    """
    return _generate_anchors(
        stride,
        np.array(sizes, dtype=np.float) / stride,
        np.array(aspect_ratios, dtype=np.float),
    )