Derive Macro SimpleForwardDiffable 

#[derive(SimpleForwardDiffable)]

implement a default ForwardDiffable with trait bounds as the following:

impl<S, SelfInput, SelfOutput, SelfGradient> autodiff::autodiffable::ForwardDiffable<S> for T
where
    Self: autodiff::autodiffable::AutoDiffable<S, Input = SelfInput, Output = SelfOutput>,
    SelfInput: autodiff::gradienttype::GradientType<SelfOutput, GradientType = SelfGradient>,
    SelfGradient: autodiff::forward::ForwardMul<SelfInput, SelfInput, ResultGrad = SelfOutput>,
{
    //type Input = SelfInput;
    //type Output = SelfOutput;
    fn eval_forward_grad(
        &self,
        x: &Self::Input,
        dx: &Self::Input,
        s: &S,
    ) -> (Self::Output, Self::Output) {
        let (f, df) = self.eval_grad(x, s);
        (f, df.forward_mul(dx))
    }

}

This will only work if:

• the struct is AutoDiffable
• the input type is GradientType<OutputType>
• the output type is ForwardMul<InputType, InputType, ResultGrad = OutputType>