Skellam (special case) is broken with Zygote

[devmotion]

Currently, these distributions are broken due to FluxML/Zygote.jl#873. As discussed in TuringLang/Bijectors.jl#155, the corresponding tests are marked as broken. The issue will be fixed upstream.

[devmotion]

The main issue was fixed upstream, only some interesting issue due to Zygote._zero with ChainRulesCore.NotImplemented for matrices of Skellam variates is left: #172