Support `Zygote.forwarddiff` & `Zygote.checkpointed`

[prbzrg]

Hi,
I have a question, do we need to support Zygote.forwarddiff? What about Zygote.checkpointed?
I think, there can be parameters of AutoZygote to activate them.

[gdalle]

• Supporting Zygote.forwarddiff seems weird to me, because if you really need to differentiate a function with ForwardDiff.jl you will use AutoForwardDiff
• Supporting Zygote.checkpointed seems reasonable, and it could be a field checkpointed::Bool of AutoZygote in the same way that e.g. AutoReverseDiff has compile::Bool. A type parameter would seem more type-stable, but given how unstable Zygote.jl itself is, I don't know if it matters. @Vaibhavdixit02, any idea?