Zygote AD failure for ChainTransform

[st--]

Currently our build is broken due to a Zygote AD issue. To unblock other PRs, let's uncomment the test (see #414) and investigate the issue separately.

[st--]

The issue seems to be that Zygote does not properly differentiate through foldl() [returning nothing] :( no idea why it's not working, there's implementations such as JuliaDiff/ChainRules.jl#526 ...

[willtebbutt]

😞