Generate anchor (reference) windows by enumerating aspect ratios X
    scales wrt a reference (0, 0, base_size - 1, base_size - 1) window.

def _generate_anchors(base_size, scales, aspect_ratios):
    """Generate anchor (reference) windows by enumerating aspect ratios X
    scales wrt a reference (0, 0, base_size - 1, base_size - 1) window.
    """
    anchor = np.array([1, 1, base_size, base_size], dtype=np.float) - 1
    anchors = _ratio_enum(anchor, aspect_ratios)
    anchors = np.vstack(
        [_scale_enum(anchors[i, :], scales) for i in range(anchors.shape[0])]
    )
    return torch.from_numpy(anchors)