doubt on the gradients

[maybeluo]

According to the models, if we use "margincost", we should get their gradients for positive and negative triplet respectively. But in the code of model.py, take #L1208 in "TrainFn1Member" for example, you add them together before compute their gradients. Because "costl" and "costr" share some varibles, I'm wondering if it can work just like the models defined.