Start replacing tests with DifferentiationInterface

[gdalle]

Following today's conversation between the maintainers of Flux and DifferentiationInterface, here's a first attempt at replacing small parts of the test suite with a more generic version.

To save CI time, I have (temporarily) deactivated all workflows except testing, and commented out all tests except the ones I'm working on.

This is a very rough draft to start the discussion, do not merge!

Files done

• softmax.jl

[gdalle]

@adrhill first roadblock with our approach: we'd like to validate the pullback of Zygote using FiniteDifferences, but as a "forward mode" backend, the latter doesn't have a natural pullback definition

[gdalle]

Ok the solution is to use https://juliadiff.org/FiniteDifferences.jl/latest/pages/api/#FiniteDifferences.j%E2%80%B2vp