Add GATConv docstring

Author: aurorarossi

GNNLux/src/layers/conv.jl

@@ -784,6 +784,69 @@ function Base.show(io::IO, l::DConv)
     print(io, "DConv($(l.in_dims) => $(l.out_dims), k=$(l.k))")
 end
 
+@doc raw"""
+    GATConv(in => out, σ = identity; heads = 1, concat = true, negative_slope = 0.2, init_weight = glorot_uniform, init_bias = zeros32, use_bias = true, add_self_loops = true, dropout=0.0)
+    GATConv((in, ein) => out, ...)
+
+Graph attentional layer from the paper [Graph Attention Networks](https://arxiv.org/abs/1710.10903).
+
+Implements the operation
+```math
+\mathbf{x}_i' = \sum_{j \in N(i) \cup \{i\}} \alpha_{ij} W \mathbf{x}_j
+```
+where the attention coefficients ``\alpha_{ij}`` are given by
+```math
+\alpha_{ij} = \frac{1}{z_i} \exp(LeakyReLU(\mathbf{a}^T [W \mathbf{x}_i; W \mathbf{x}_j]))
+```
+with ``z_i`` a normalization factor. 
+
+In case `ein > 0` is given, edge features of dimension `ein` will be expected in the forward pass 
+and the attention coefficients will be calculated as  
+```math
+\alpha_{ij} = \frac{1}{z_i} \exp(LeakyReLU(\mathbf{a}^T [W_e \mathbf{e}_{j\to i}; W \mathbf{x}_i; W \mathbf{x}_j]))
+```
+
+# Arguments
+
+- `in`: The dimension of input node features.
+- `ein`: The dimension of input edge features. Default 0 (i.e. no edge features passed in the forward).
+- `out`: The dimension of output node features.
+- `σ`: Activation function. Default `identity`.
+- `heads`: Number attention heads. Default `1`.
+- `concat`: Concatenate layer output or not. If not, layer output is averaged over the heads. Default `true`.
+- `negative_slope`: The parameter of LeakyReLU.Default `0.2`.
+- `init_weight`: Weights' initializer. Default `glorot_uniform`.
+- `init_bias`: Bias initializer. Default `zeros32`.
+- `use_bias`: Add learnable bias. Default `true`.
+- `add_self_loops`: Add self loops to the graph before performing the convolution. Default `true`.
+- `dropout`: Dropout probability on the normalized attention coefficient. Default `0.0`.
+
+# Examples
+
+```julia
+using GNNLux, Lux, Random
+
+# initialize random number generator
+rng = Random.default_rng()
+
+# create data
+s = [1,1,2,3]
+t = [2,3,1,1]
+in_channel = 3
+out_channel = 5
+g = GNNGraph(s, t)
+x = randn(rng, Float32, 3, g.num_nodes)
+
+# create layer
+l = GATConv(in_channel => out_channel; add_self_loops = false, use_bias = false, heads=2, concat=true)
+
+# setup layer
+ps, st = LuxCore.setup(rng, l)
+
+# forward pass
+y, st = l(g, x, ps, st)       
+```
+"""
 @concrete struct GATConv <: GNNLayer
     dense_x
     dense_e