Returns:
        label_logits: fHxfWxNA
        box_logits: fHxfWxNAx4

def rpn_head(featuremap, channel, num_anchors):
    """
    Returns:
        label_logits: fHxfWxNA
        box_logits: fHxfWxNAx4
    """
    with argscope(Conv2D, data_format='channels_first',
                  kernel_initializer=tf.random_normal_initializer(stddev=0.01)):
        hidden = Conv2D('conv0', featuremap, channel, 3, activation=tf.nn.relu)

        label_logits = Conv2D('class', hidden, num_anchors, 1)
        box_logits = Conv2D('box', hidden, 4 * num_anchors, 1)
        # 1, NA(*4), im/16, im/16 (NCHW)

        label_logits = tf.transpose(label_logits, [0, 2, 3, 1])  # 1xfHxfWxNA
        label_logits = tf.squeeze(label_logits, 0)  # fHxfWxNA

        shp = tf.shape(box_logits)  # 1x(NAx4)xfHxfW
        box_logits = tf.transpose(box_logits, [0, 2, 3, 1])  # 1xfHxfWx(NAx4)
        box_logits = tf.reshape(box_logits, tf.stack([shp[2], shp[3], num_anchors, 4]))  # fHxfWxNAx4
    return label_logits, box_logits