implement ResGatedGraphConv and support Parallel in GNNChain

[CarloLucibello]

cc @r3tex

[codecov]

Codecov Report

Merging #46 (1bfb633) into master (3585cca) will decrease coverage by 0.75%.
The diff coverage is 70.27%.

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
- Coverage   79.82%   79.06%   -0.76%     
==========================================
  Files           7        7              
  Lines         570      602      +32     
==========================================
+ Hits          455      476      +21     
- Misses        115      126      +11     

Impacted Files	Coverage Δ
src/layers/basic.jl	39.13% <33.33%> (-0.87%)	⬇️
src/layers/conv.jl	72.22% <72.72%> (-0.66%)	⬇️
src/utils.jl	100.00% <100.00%> (ø)

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[r3tex]

FYI - you included double parentheses around the link to the paper so it doesn't redirect correctly.