`test_rrule` trying to find `Tangent` on `axes`

[theogf]

This is more a question than a bug report.

I defined the following rrule for Fill with _map (Zygote does not allow playing with map :) )

_map(f, args...) = map(f, args...)
function ChainRulesCore.rrule(config::RuleConfig{>:HasReverseMode}, ::typeof(_map), f::Tf, x::F) where {Tf,F<:Fill}
    y_el, back = ChainRulesCore.rrule_via_ad(config, f, x.value)
    function _map_Fill_rrule(Δ)
        Δf, Δx_el = back(Δ.value)
        return NoTangent(), Δf, Tangent{F}(value = Δx_el, axes = NoTangent())
    end
    return Fill(y_el, axes(x)), _map_Fill_rrule
end

The result seems correct but I cannot call test_rrule on it:

test_rrule(_map, sum, Fill(randn(3, 4), 4))

The error narrows down to the jacobian function from FiniteDifferences trying to differentiate through the axes field of Fill.

I tried to pass a Tangent to Fill via ⊢ Tangent{typeof(x)}(value=randn(3, 4), axes=NoTangent()) but without success...

Could you help me figure out what I need to do?

[theogf]

Nevermind dispatching on to_vec solves the issue as proposed in #258