Validate which keypoints are contained inside a given box.
    points: NxKx2
    boxes: Nx4
    output: NxK

def _within_box(points, boxes):
    """Validate which keypoints are contained inside a given box.
    points: NxKx2
    boxes: Nx4
    output: NxK
    """
    x_within = (points[..., 0] >= boxes[:, 0, None]) & (
        points[..., 0] <= boxes[:, 2, None]
    )
    y_within = (points[..., 1] >= boxes[:, 1, None]) & (
        points[..., 1] <= boxes[:, 3, None]
    )
    return x_within & y_within