Missing rule from cumprod

[longemen3000]

i cant find the cumprod rules, and the second paragraph of https://discourse.julialang.org/t/why-does-zygote-produce-a-wrong-derivative/45724 makes me suspect that there isn't

[mcabbott]

xref FluxML/Zygote.jl#282 . The tricky part is making it handle zero entries correctly.