Use rrule of KwFunc for Core.kwcall

[nmheim]

This seems to fix non-differentiable keyword arguments by constructing the KwFunc defined in Diffractor.

[oxinabox]

why isn't this

Suggested change

	rrule(KwFunc(f), kwargs, f, args...)
	rrule(f, args...; kwargs...)

is that the same, or is it different?

[nmheim]

Because (I think) we want to hit this rrule for KwFunc:
https://github.com/JuliaDiff/Diffractor.jl/pull/270/files#diff-5bd76352c0319c1d2659edc9825a90b92ac204aa1c17b9b95a3880e0fd98855eR255

[nmheim]

I am not exactly sure why the KwFunc struct is needed though.. it seems like could be done via rrule(::typeof(Core.kwcall), kwargs, f, args...) directly?

[nmheim]

Removing the KwFunc and dispatching on kwcall directly seems to work, but I was afraid to remove something which I don't exactly understand

function ChainRulesCore.rrule(::typeof(Core.kwcall), kwargs, f, args...)
    r = Core.kwfunc(rrule)(kwargs, rrule, f, args...)
    if r === nothing
        return nothing
    end
    x, back = r
    x, Δ->begin
        (NoTangent(), NoTangent(), back(Δ)...)
    end
end

[oxinabox]

Oh this might be the thing that is there to avoid ADing through so much of the kwarg machinery in the nested AD case

[oxinabox]

@Keno I think this does not have the same concerns that #266 has, but I am not sure.
Because for reverse mode everything is already bad.

Should we do this, and reconsider it later as needed?
It unbreaks a lot of tests

[Keno]

Same as in #266 I think in order for this not to make inference worse, the method should be split into kw and non-kw versions.

[nmheim]

its basically a copy of the non-kw version of the function, but that is what we want in order to avoid ADing through the kw machinery, if there are no kws if I understand correctly?