batching scales quadratically

[tclements]

GraphNeuralNetworks.batch scales quadratically in run time and memory use with the number of graphs to batch. Here is a MWE with benchmarks:

using BenchmarkTools
using GraphNeuralNetworks

g1 = rand_graph(4, 6, ndata=ones(8, 4));
g2 = rand_graph(7, 4, ndata=zeros(8, 7));
GraphNeuralNetworks.batch([g1, g2]);

for ngraphs in 2 .^ (8:10)
    gs = [rand_graph(4, 6, ndata=ones(8, 4)) for _ in 1:ngraphs]
    println("\n=======================\nBatchsize = $ngraphs graphs\n=======================\n")    
    b = @benchmark GraphNeuralNetworks.batch($gs)
    display(b)
end

=======================
Batchsize = 256 graphs
=======================

BenchmarkTools.Trial: 1532 samples with 1 evaluation.
 Range (min … max):  2.583 ms … 6.482 ms  ┊ GC (min … max):  0.00% … 54.02%
 Time  (median):     2.712 ms             ┊ GC (median):     0.00%
 Time  (mean ± σ):   3.260 ms ± 1.278 ms  ┊ GC (mean ± σ):  16.20% ± 19.62%

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  ████▃▂▂▁▁▁▁▂▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▃▄▄▄▃ ▂
  2.58 ms        Histogram: frequency by time       6.33 ms <

 Memory estimate: 13.08 MiB, allocs estimate: 17790.

=======================
Batchsize = 512 graphs
=======================

BenchmarkTools.Trial: 510 samples with 1 evaluation.
 Range (min … max):   8.219 ms … 11.230 ms  ┊ GC (min … max):  0.00% … 21.96%
 Time  (median):     10.632 ms              ┊ GC (median):    20.63%
 Time  (mean ± σ):    9.790 ms ±  1.241 ms  ┊ GC (mean ± σ):  13.99% ± 10.61%

  ▁█                                                    ▁▂▁
  ███▆▄▂▂▁▁▁▁▁▁▂▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▂▄▆▆▄▄▆███▅▄ ▃
  8.22 ms         Histogram: frequency by time        11.1 ms <

 Memory estimate: 50.14 MiB, allocs estimate: 36053.

=======================
Batchsize = 1024 graphs
=======================

BenchmarkTools.Trial: 150 samples with 1 evaluation.
 Range (min … max):  32.179 ms … 37.269 ms  ┊ GC (min … max): 11.89% … 16.35%
 Time  (median):     32.532 ms              ┊ GC (median):    12.07%
 Time  (mean ± σ):   33.527 ms ±  1.463 ms  ┊ GC (mean ± σ):  12.63% ±  1.19%

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  ████▅▄▅▃▃▃▃▁▁▁▁▃▃▁▁▁▁▁▃▃▃▁▃▃▁▃▄▃▆▃▅▃▄▃▃▄▅▃▃▁▁▄▁▃▁▁▃▁▁▁▃▃▃▁▃ ▃
  32.2 ms         Histogram: frequency by time          37 ms <

 Memory estimate: 196.20 MiB, allocs estimate: 73941.

I came up with a fix to the current implementation and will submit a PR soon.