Change Zygote.forwarddiff to ZygoteRules.forwarddiff?

[oschulz]

Would it be possible to have the forwarddiff wrapper/hint in ZygoteRules instead of Zygote, to allow developers to include mixed-mode-diff hinting in functions without depending on Zygote?

[oschulz]

And maybe the same for Zygote.ignore?

[MikeInnes]

I think ZygoteRules should really be the bare minimum to add rules to Zygote, otherwise it will discourage people from depending on it. But perhaps we could have something like a ZygoteUtils package that has these in.

[oschulz]

But ignore and forwarddiff are extremely lightweight, right? Just the definition, I mean?

[oschulz]

But based on the discussion in FluxML/Zygote.jl#716, I guess ChainRulesCore would probably the best place for it, so users can prepare their functions for autodiff in an engine-independent way? I'll close this issue here.