dynamic NaN-safe mode switching

[jrevels]

Follow-up issue to #179. From my comment there:

In a future PR, we could add AbstractConfig constructors like GradientConfig(x, Chunk{N}, NaNSafe{true}). The generated dual numbers would then contain partials of type Partials{N,T,true}, which would dispatch to the NaN-safe versions of methods. Downstream modules could then support passing that option through their APIs (or simply setting it to a reasonable default for themselves internally).

[cossio]

This would be very nice to have!

[taylormcd]

Is it possible we could use Preferences.jl to enable/disable NaN-safe mode now? See https://github.com/JuliaLang/Pkg.jl/issues/458. I think that would effectively solve this long-standing issue.

[devmotion]

NaN-safe mode switching via the Preferences.jl system was added in https://github.com/JuliaDiff/ForwardDiff.jl/pull/539.