Created by: wangkuiyi

The LaTeX equation of the operator HuberLoss isn't encapsulated by a pair of double-dollar $$:

https://github.com/PaddlePaddle/Paddle/blob/c0d2ca54b9bfea943c61ae09573ee188e0e1042b/paddle/operators/huber_loss_op.cc#L68-L79

Please add them, while taking the following code as an example:

https://github.com/PaddlePaddle/Paddle/blob/d883547bf072f1edd05eb52854e56970b50e7203/paddle/operators/lrn_op.cc#L89-L96