Export AbstractDifferential?

[mattBrzezinski]

Stemming from: JuliaDiff/ChainRulesTestUtils.jl#6 (comment)

Is there a reason why we don't export AbstractDifferential? Would it be useful for other packages (other than ChainRulesTestUtils.jl?)

[oxinabox]

I am not sure this type should even exist.
Not all differentials are subtypes of AbstractDifferential (some are natural),
And some subtypes of AbstractDifferential are also used as scale factors (Zero, One).

And we have few good methods on it

[nickrobinson251]

I'm not sure the one use of it in ChainRulesTestUtils is good either JuliaDiff/ChainRulesTestUtils.jl#7