code: public double initFNum(double w, double o, double y) {
      switch (distribution) {
        case AUTO:
        case gaussian:
        case bernoulli:
        case quasibinomial:
        case multinomial:
          return w*(y-o);
        case poisson:
          return w*y;
        case gamma:
          return w*y*linkInv(-o);
        case tweedie:
          return w*y*exp(o*(1- tweediePower));
        case modified_huber:
          return y==1 ? w : 0;
        default:
          throw H2O.unimpl();
      }
    }
docs: Contribution to numerator for initial value computation
  @param w weight
  @param o offset
  @param y response
  @return weighted contribution to numerator