Implement Functors.functor

[devmotion]

This PR is an alternative to #161. As mentioned there, Flux.@treelike does not exist anymore. Instead, Flux relies on Functors.functor and reexports Flux.@functor.

This PR implements Functors.functor for bijectors and normalizing flows. Thus users can just call Flux.params without implementing @functor for the different layers.

A dependency on the lightweight package Functors.jl is added and the optional dependency on Flux is dropped.

[torfjelde]

This is great stuff!
I have one comment about the testing; it seems like we currently only test a couple of the bijectors?

[torfjelde]

Is this being used anywhere?

[devmotion]

Yes, in the norm_flows tests.

[torfjelde]

Ah, missed that 👍

[devmotion]

Yes, indeed, so far only InvertibleBatchnorm, PlanarLayer, and RadialLayer are tested in norm_flows.jl. I am still a bit confused about the test setup - where would be the best place to add tests for other bijectors? Initially I assume that most interface tests would be handled by test_bijector but it seems it is only used in one single test?

[torfjelde]

where would be the best place to add tests for other bijectors? Initially I assume that most interface tests would be handled by test_bijector but it seems it is only used in one single test?

That's correct, but the idea is to start using that approach for all bijectors. So feel free to add it there for now :)

[torfjelde]

Actually, I guess it's somewhat non-trivial to add this to test_bijector, as we don't have access to the parameters of the bijector itself?

[devmotion]

I assume one would have to specify the parameters explicitly, similar to test_functor. However, one could set them to nothing by default and skip the tests in this case.

[torfjelde]

E.g.

function test_bijector(
    b::Bijector,
    xs_true,
    ys_true,
    logjacs_true
    params = nothing;
    kwargs...
)
    !isnothing(params) && test_functor(b, params)
    return test_bijector(b, xs_true, ys_true, logjacs_true; kwargs...)
end

?

[devmotion]

This will lead to a stackoverflow error. But in principle this is the idea.

[torfjelde]

Oh yeah, rather:

function test_bijector(
    b::Bijector,
    xs_true,
    ys_true,
    logjacs_true
    params;
    kwargs...
)
    test_functor(b, params)
    return test_bijector(b, xs_true, ys_true, logjacs_true; kwargs...)
end

[torfjelde]

I'm happy for now, and we can make it testing the functor functionality part of the tests once we unify testing a bit.

[devmotion]

Ah sorry, I forgot about the PR. OK, let's update the tests separately then!