Add topk features

[aurorarossi]

With this PR I add the topk_nodes and topk_edges features (see issue https://github.com/CarloLucibello/GraphNeuralNetworks.jl/issues/41).

[CarloLucibello]

I think it is more useful and general to have a function with signature

topk_nodes(g::GNNGraph, x::AbstractArray, k::Int; rev::Bool = true, sortby::Union{Nothing, Int} = nothing)

instead of passing the tensor name.

Also, the docstring needs much more explanation of the purpose of this function, the various arguments, and the returned objects.

Following DGL, I think it is useful to return both the sorted array and permutation. So if we call (y, partialperm) the output:

• y should be an array of size (num_feat, k, num_graphs)
• partialperm should be an array of integers of size (k, num_graphs) if sortby === nothing and of size (num_feat, k, num_graphs) if sortby is an integer.

[CarloLucibello]

Does it work on gpu as well?

[aurorarossi]

Yes, it works on the GPU.

[CarloLucibello]

I think it would be nice to contribute topk_matrix as a topk function in MLUtils.jl and then base this PR on top of that.

Then I would have here two functions, topk_nodes and topk_edges as in Deep Graph Library (https://docs.dgl.ai/generated/dgl.topk_nodes.html#dgl.topk_nodes)