Support Forward Mode of `Zygote`

[prbzrg]

It seems that Zygote support forward AD but a few use that. Is there any plan to support it?
I opened this issue to ask for comments.

Related API:

• Zygote.pushforward
• Zygote.forwarddiff

Related Links:
https://github.com/FluxML/Zygote.jl/tree/master/src/forward

https://github.com/FluxML/Zygote.jl/tree/master/test/forward

FluxML/Zygote.jl#503

FluxML/Zygote.jl#1098

[adrhill]

If this forward mode is feature-complete, the first step is to add it to ADTypes. Then DI can support both modes the same way it does with Enzyme.

[gdalle]

It's not public and it's barely used so we won't touch it