Added TransformerConv usage example (#422)

* transformerconv usage example

* fix

* Update src/layers/conv.jl

Co-authored-by: Carlo Lucibello <[email protected]>

---------

Co-authored-by: Carlo Lucibello <[email protected]>

Author: rbSparky

src/layers/conv.jl

@@ -1790,6 +1790,18 @@ can be performed.
 - `ff_channels`: If positive, a feed-forward NN is appended, with the first having the given
     number of hidden nodes; this NN also gets a skip connection and batch normalization 
     if the respective parameters are set. Default: `0`.
+
+# Examples
+
+```julia
+N, in_channel, out_channel = 4, 3, 5
+ein, heads = 2, 3
+g = GNNGraph([1,1,2,4], [2,3,1,1])
+l = TransformerConv((in_channel, ein) => in_channel; heads, gating = true, bias_qkv = true)
+x = rand(Float32, in_channel, N)
+e = rand(Float32, ein, g.num_edges)
+l(g, x, e)
+```        
 """
 struct TransformerConv{TW1, TW2, TW3, TW4, TW5, TW6, TFF, TBN1, TBN2} <: GNNLayer
     W1::TW1