Fix TrackedReal nextfloat and prevfloat

[avik-pal]

As discussed with @DhairyaLGandhi it would make sense to move these functions to Tracker.jl

[yebai]

It's a good idea to consider transferring Tracker.jl specific rules back to Tracker.jl, perhaps under a dedicated folder for Distributions.jl. More generally, we should consider transferring implementations related to Zygote.jl, ReverseDiff.jl and ForwardDiff.jl back to their main repo as well.

However, we don't currently have merge access for repos mentioned above. It would be helpful if some Turing members (e.g. @mohamed82008, @willtebbutt, @devmotion and @yebai) are allowed to maintain these repos such that bug fixes can be merged quickly.

[willtebbutt]

I would not be pro- this happening. Now that JuliaDiff/ChainRulesCore.jl#182 is in, Tracker and ReverseDiff should join Zygote and move over to ChainRules.

The @adjoints currently implemented should be replaced with rrules.

Tracker- and ReverseDiff-specific rules should be removed.

[mohamed82008]

Ya I agree with Will, ChainRules is the future. May not be worth moving things around now unless they are urgently needed by users who don't want to load DistributionsAD.

[yebai]

Maintaining so many AD libraries is probably not the most fabulous idea in my view. In particular, Tracker and ReverseDiff are the same apart from implementation details.