说明：  这个是用于深度度量学习的损失函数，是百度提出来的，一个别出心裁的方法，别人都用距离来度量样本间的差异，作者使用角度来度量的
(This is a loss function for in-depth measurement of learning. It is proposed by Baidu. It is an ingenious method. Others use distance to measure the difference between samples, and the author uses angle to measure it)