how to query all supported interfaces

[tpapp]

For unit testing, it may be convenient to get a list of all supported interfaces. We may just maintain a list like

const AD_INTERFACES = [:ForwardDiff, :ReverseDiff, :Zygote, :Tracker, :Enzyme]

but it may be better to have a function

function AD_gradients_for_CI(ℓ; options...) # options to be clarified

end

that returns a list, so that we can do all variants, eg Reverse AD with and without tape, etc.