[WIP] [GNNLux] Adding TransformerConv Layer

[rbSparky]

Adding conv layer according to #461

[rbSparky]

tests work with skip connection false

but fail with skip connection true

dimension error:

Error in testset "TransformerConv" on worker 2157:
Error During Test at /home/runner/work/GraphNeuralNetworks.jl/GraphNeuralNetworks.jl/GNNLux/test/layers/conv_tests.jl:8
Got exception outside of a @test
DimensionMismatch: A has shape (16, 16) but B has shape (6, 10)

will also fix the GNN test since that is due to change of variable name

[rbSparky]

@CarloLucibello let me know if you have ideas to fix with skip connection
I will look further into it too