Unthunk each element in ∇eachslice

[pxl-th]

This is required to make Zygote work with thunks (using this PR FluxML/Zygote.jl#966).

MWE:

x = ones(Float32, 3, 2)
Zygote.gradient(x) do x
    sum(map(norm, eachcol(x)))
end

Returns:

(Float32[0.0 0.0; 0.0 0.0; 0.0 0.0],)

The reason this happens is because ZBack no longer unthunks Thunks when being called (in wrap_chainrules_output), so rrule for eachcol receives dy as:

Vector{ChainRulesCore.InplaceableThunk{ChainRulesCore.Thunk{ChainRules.var"#1071#1074"{Float32, SubArray{Float32, 1, Matrix{Float32}, Tuple{Base.Slice{Base.OneTo{Int64}}, Int64}, true}, Float32}}, ChainRules.var"#1070#1073"{Float32, SubArray{Float32, 1, Matrix{Float32}, Tuple{Base.Slice{Base.OneTo{Int64}}, Int64}, true}, Float32}}}

Where each element of Vector is produced by norm_pullback_2.
While previously due to unthunking in ZBack we'd get for dy:

Vector{Vector{Float32}}

And so the execution of ∇eachslice woudn't terminate here.