Loss functions

VW currently supports these loss functions:

1. Squared loss [\ell(p,y)=\frac{1}{2}(p-y)^2]
2. Logistic loss [\ell(p,y)=\log(1+\exp(-yp))]
3. Hinge loss [\ell(p,y)=\max(0,1-yp)]
4. (\tau)-Quantile loss [\ell(p,y)=\tau(p-y)\mathbb{I}(y<p) +(1-\tau)(y-p)\mathbb{I}(y \geq p) ]