Macro create_custom_autograd_function 

macro_rules! create_custom_autograd_function {
    (
        name: $name:ident,
        inputs: $num_inputs:expr,
        outputs: $num_outputs:expr,
        forward: |$inputs:ident| $forward_body:expr,
        backward: |$grad_outputs:ident, $backward_inputs:ident| $backward_body:expr
    ) => { ... };
}

Macro for creating simple custom autograd functions