Backend switching for ReverseDiff.jl

Like https://github.com/JuliaDiff/DifferentiationInterface.jl/pull/768 but ReverseDiff.jl. 

See the `ReverseDiff.@grad` [implementation](https://github.com/JuliaDiff/ReverseDiff.jl/blob/b73cb62e6c187ef62e41190b6f02b53e2678df9e/src/macros.jl#L179) for how to define new rules for ReverseDiff.