Fix from_networkx in case where attributes are tensors

[Padarn]

Fixes problem reported in #4445, the problem being that

torch.tensor([torch.Tensor([1,2,3], torch.Tensor([1,2,3]])

does not work, they need to be stacked instead.

[codecov]

Codecov Report

Merging #4486 (d45ab6e) into master (e18198f) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

❗ Current head d45ab6e differs from pull request most recent head a144f8b. Consider uploading reports for the commit a144f8b to get more accurate results

@@            Coverage Diff             @@
##           master    #4486      +/-   ##
==========================================
- Coverage   82.75%   82.63%   -0.12%     
==========================================
  Files         313      313              
  Lines       16261    16262       +1     
==========================================
- Hits        13456    13438      -18     
- Misses       2805     2824      +19     

Impacted Files	Coverage Δ
torch_geometric/utils/convert.py	85.91% <100.00%> (+0.30%)	⬆️
torch_geometric/nn/conv/utils/typing.py	81.25% <0.00%> (-17.50%)	⬇️
torch_geometric/io/tu.py	93.58% <0.00%> (-2.57%)	⬇️
torch_geometric/nn/models/mlp.py	98.41% <0.00%> (-1.59%)	⬇️
torch_geometric/transforms/gdc.py	78.17% <0.00%> (-1.02%)	⬇️
torch_geometric/data/dataset.py	96.80% <0.00%> (-0.80%)	⬇️
torch_geometric/nn/conv/rgat_conv.py	83.76% <0.00%> (-0.53%)	⬇️
torch_geometric/graphgym/utils/comp_budget.py	15.51% <0.00%> (+0.51%)	⬆️

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[rusty1s]

Thank you!