Add GNAN model to PyG

[codearky]

This PR adds the GNAN (https://arxiv.org/pdf/2406.01317) model and a dedicated dataloader to PyG

[codecov]

Codecov Report

❌ Patch coverage is 94.02174% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.01%. Comparing base (c211214) to head (dcbde24).
⚠️ Report is 98 commits behind head on master.

Files with missing lines	Patch %	Lines
torch_geometric/nn/models/gnan.py	92.48%	10 Missing ⚠️
torch_geometric/loader/gnan_dataloader.py	98.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10371      +/-   ##
==========================================
- Coverage   86.11%   86.01%   -0.10%     
==========================================
  Files         496      504       +8     
  Lines       33655    35391    +1736     
==========================================
+ Hits        28981    30442    +1461     
- Misses       4674     4949     +275     

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[puririshi98]

LGTM at a high level, but please lets move the example to examples/explain.
Please also add a sentence to examples/README.md to mention, "For examples on explainability with GNNs, see examples/explain."
Then please also update examples/explain/README.md to have a short explanation of what the new example does.
Once thats done I will do a deeper review and can merge