在keras中实现三重损失的准确性

import tensorflow as tf
from keras import backend as K

def triplet_loss(y_true, y_pred, alpha=0.2):
    anchor, positive, negative = y_pred[:, 0], y_pred[:, 1], y_pred[:, 2]
    pos_dist = K.sum(K.square(anchor - positive), axis=-1)
    neg_dist = K.sum(K.square(anchor - negative), axis=-1)
    basic_loss = pos_dist - neg_dist + alpha
    loss = K.maximum(basic_loss, 0.0)
    return loss

model.compile(loss=triplet_loss, optimizer='adam')