Fix GraphKernel

[yuehhua]

Closes #74

[codecov]

Codecov Report

Merging #75 (6cbc9e8) into master (d6dbe6b) will increase coverage by 0.16%.
The diff coverage is 100.00%.

❗ Current head 6cbc9e8 differs from pull request most recent head cf34d62. Consider uploading reports for the commit cf34d62 to get more accurate results

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
+ Coverage   94.48%   94.65%   +0.16%     
==========================================
  Files           9        8       -1     
  Lines         127      131       +4     
==========================================
+ Hits          120      124       +4     
  Misses          7        7              

Impacted Files	Coverage Δ
src/operator_kernel.jl	100.00% <100.00%> (ø)
src/Transform/fourier_transform.jl	100.00% <0.00%> (ø)
src/NOMAD/NOMAD.jl
src/FNO/FNO.jl
src/DeepONet/subnets.jl
src/graph_kernel.jl
src/DeepONet/DeepONet.jl
src/model.jl	100.00% <0.00%> (ø)
src/subnets.jl	100.00% <0.00%> (ø)
... and 2 more

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[yuehhua]

It needs support of positional encoding as edge feature to feed into the model. It may need some design from GeometricFlux.

[yuehhua]

Maybe fix GraphKernel layer first. I will fix the example later.

[foldfelis]

Wow~~