Function reverse_partials 

pub fn reverse_partials<T: Float>(op: OpCode, a: T, b: T, r: T) -> (T, T)

Available on crate feature bytecode only.

Compute reverse-mode partial derivatives for a single opcode.

Returns (∂result/∂arg0, ∂result/∂arg1). For unary ops the second partial is T::zero().

a, b are the operand values (at recording or re-evaluation time). r is the result value.

Generic over T: Float so Phase 3 can call it with Dual<F> for forward-over-reverse.