fix GNNChain restructure bug

[mplemay]

Hi this is my attempted fix for #200. Please let me know if there is anything I can do to improve this PR.

[codecov]

Codecov Report

Merging #201 (c4d562c) into master (401ae1a) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
- Coverage   86.56%   86.44%   -0.12%     
==========================================
  Files          15       15              
  Lines        1347     1365      +18     
==========================================
+ Hits         1166     1180      +14     
- Misses        181      185       +4     

Impacted Files	Coverage Δ
src/layers/basic.jl	50.00% <100.00%> (+1.21%)	⬆️
src/GNNGraphs/gnngraph.jl	72.50% <0.00%> (-2.50%)	⬇️
src/layers/conv.jl	77.23% <0.00%> (-0.08%)	⬇️
src/GNNGraphs/sampling.jl	100.00% <0.00%> (ø)
src/GNNGraphs/query.jl	93.67% <0.00%> (+0.03%)	⬆️
src/GNNGraphs/transform.jl	96.96% <0.00%> (+0.05%)	⬆️
src/GNNGraphs/convert.jl	90.40% <0.00%> (+0.31%)	⬆️

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[CarloLucibello]

thank you for this patch!