Add rrule for NamedTuple merge

[oxinabox]

Based on LuxDL/Lux.jl#659
but with some fixes up to use ChainRulesCore types.

Initial PR is not inferable.
I suspect i will need to rewrite this as a generated funcition to make it infer.

@avik-pal do you want to test this branch against your use case, with it deleted from Lux?

[oxinabox]

What i want is to do

_getproperty_kwexpr(key) = :($key = getproperty(dy, $key))
@generated function rrule(::typeof(merge), nt1::NamedTuple{F1}, nt2::NamedTuple{F2}) where {F1, F2}
    quote
        y = merge(nt1, nt2)
        function merge_pullback(dy)
            dnt1 = Tangent{typeof(nt1)}(; $(map(_getproperty_kwexpr, setdiff(F1, F2))...))
            dnt2 = Tangent{typeof(nt2)}(; $(map(_getproperty_kwexpr, F2)...))
            return (NoTangent(), dnt1, dnt2)
        end
        merge_pullback(dy::AbstractThunk) = merge_pullback(unthunk(dy))
        merge_pullback(x::AbstractZero) = (NoTangent(), x, x)
        
        return y, merge_pullback
    end
end

But generated functions are not allowed to have bodys that create closures

[oxinabox]

Final solution was tricky to get to but clean

[oxinabox]

@avik-pal can you review?

[avik-pal]

Let me trigger the tests for SciMLSensitivity with this PR

[avik-pal]

@oxinabox this should be good to go, downstream tests work