code: @Override
      public VarTensor forward() {
          MVecArray<VarTensor> xs = modIn.getOutput();
          y = new VarTensor(s, new VarSet(), s.one());
          for (int a=0; a<xs.dim(); a++) {
              VarTensor x = xs.get(a);
              y.prod(x);
          }
          return y;
      }
docs: Forward pass: p(y) = \prod_a \psi(y_a) where y_a is a {@link VarTensor} and we use variable-tensor product.