remove cast rrule not needed anymore

[CarloLucibello]

Thanks to FluxML/Zygote.jl#1171

[codecov]

Codecov Report

Merging #135 (2be04f3) into master (dd4a54c) will increase coverage by 0.49%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
+ Coverage   84.94%   85.43%   +0.49%     
==========================================
  Files          15       15              
  Lines        1262     1243      -19     
==========================================
- Hits         1072     1062      -10     
+ Misses        190      181       -9     

Impacted Files	Coverage Δ
src/utils.jl	100.00% <ø> (+10.86%)	⬆️
src/GNNGraphs/utils.jl	74.00% <0.00%> (-1.25%)	⬇️
src/GNNGraphs/convert.jl	90.08% <0.00%> (-0.32%)	⬇️
src/GNNGraphs/query.jl	92.98% <0.00%> (-0.05%)	⬇️
src/GNNGraphs/sampling.jl	100.00% <0.00%> (ø)
src/layers/conv.jl	79.52% <0.00%> (+0.17%)	⬆️
src/GNNGraphs/transform.jl	96.71% <0.00%> (+0.43%)	⬆️
src/GNNGraphs/gnngraph.jl	77.61% <0.00%> (+3.32%)	⬆️

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[CarloLucibello]

julia 1.6 failure just a sporadic GATvs test failure