DataLoader and LearnBase compatibility of GNNGraph

[CarloLucibello]

Each graph in a batch GNNGraph is considered an observation

[codecov]

Codecov Report

Merging #22 (dd0b11d) into master (b5512db) will increase coverage by 0.18%.
The diff coverage is 93.54%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   76.01%   76.20%   +0.18%     
==========================================
  Files           7        7              
  Lines         492      500       +8     
==========================================
+ Hits          374      381       +7     
- Misses        118      119       +1     

Impacted Files	Coverage Δ
src/layers/pool.jl	94.73% <ø> (ø)
src/gnngraph.jl	85.56% <80.00%> (-0.48%)	⬇️
src/msgpass.jl	96.77% <100.00%> (ø)
src/graph_conversions.jl	55.29% <0.00%> (+1.17%)	⬆️

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