fix: Gate empty result in GATConv

[lenianiva]

Resolves #637

Fixes 3 problems:

1. In gat_conv during backpropagation, "DimensionMismatch: arrays could not be broadcast to a common size". This could be solved by removing the conditional used to calculate Wxi, Wxj:

    Wxj = l.dense_x(xj)
    Wxj = reshape(Wxj, chout, heads, :)
    Wxi = l.dense_x(xi)
    Wxi = reshape(Wxi, chout, heads, :)

2. The reshape(x, :, size(x, 3)) in gat_conv creates incompatible sizes when x is empty. This PR determines the size using the first two axes of x instead.
3. The empty array problem DimensionMismatch: variable with size(x) == (1, 1, 0) cannot have a gradient with size(dx) == (4, 1, 0). This could be fixed by a patch for ChainRulesCore.jl fix: Allow arbitrary reshape in projection if array is zero sized JuliaDiff/ChainRulesCore.jl#702

Test script:

using GNNGraphs, GraphNeuralNetworks, NNlib, Flux

graph = GNNHeteroGraph(
    Dict(
        (:A, :a, :B) => ([1, 2], [3, 4]),
        (:B, :a, :A) => ([1], [2]),
        (:C, :a, :A) => (Int[], Int[]),
        (:A, :a, :C) => (Int[], Int[]),
        (:D, :a, :A) => (Int[], Int[]),
        (:E, :a, :A) => (Int[], Int[]),
        (:E, :a, :D) => (Int[], Int[]),
        (:D, :a, :E) => (Int[], Int[]),
    );
    num_nodes = Dict(
        :A => 3, :B => 5,
        :C => 7, :D => 0, :E => 0,
    )
)

width = 9
layer = HeteroGraphConv(
    [
        (src, edge, dst) => GATConv(width => width, NNlib.elu; dropout = Float32(0.25), add_self_loops = true) for
        (src, edge, dst) in keys(graph.edata)
    ];
)
layer2 = HeteroGraphConv(
    [
        (src, edge, dst) => GATConv(width => width, NNlib.elu; dropout = Float32(0.25), add_self_loops = true) for
        (src, edge, dst) in keys(graph.edata)
    ];
)

x = (
    A = rand(Float32, width, 3),
    B = rand(Float32, width, 5),
    C = rand(Float32, width, 7),
    D = rand(Float32, width, 0),
    E = rand(Float32, width, 0),
)

x1 = layer(graph, x)
x2 = layer2(graph, x1)
@info "$x2"

g = Flux.gradient(x) do x
    y = layer(graph, x)
    sum(y[:A])
end

[lenianiva]

How would this work if bias is set to false?

Also, we need the same patch on gatv2. Simply adding a new conditional in gat_conv only solves the solution for inference. During training it does not work for some reason.

[CarloLucibello]

Suggested change

	Wxj = l.dense_x(xj)
	Wxj = reshape(Wxj, chout, heads, :)
	Wxi = l.dense_x(xi)
	Wxi = reshape(Wxi, chout, heads, :)
	Wxj = l.dense_x(xj)
	Wxj = reshape(Wxj, chout, heads, :)
	if xi !== xj
	Wxi = l.dense_x(xi)
	Wxi = reshape(Wxi, chout, heads, :)
	else
	Wxi = Wxj
	end

would work?

[lenianiva]

This doesn't work. It seems like the above example triggers both branches and Zygote gets one branch confused for another. I have seen this kind of behaviour before with Zygote.

[CarloLucibello]

regarding the wrong gradient shape for empty array, do you have any clue why it is happening? ideally the chainrule's projection shouldn't be patched, it should just receive a dx in the correct shape.

[lenianiva]

regarding the wrong gradient shape for empty array, do you have any clue why it is happening? ideally the chainrule's projection shouldn't be patched, it should just receive a dx in the correct shape.

I have absolutely no idea. A hint there is the error only occurs for 0-sized arrays. I think it has something to do with computing gradients and is outside the scope of this package.