added log_softmax to utils with tests

[elilaird]

Add log_softmax for Sparse Tensors in torch_geometric

• Implements log_softmax function for numerical stability in GNNs
• Includes comprehensive tests for correctness and backward compatibility
• Benchmarks performance against PyTorch's built-in log_softmax
• Detailed documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.59%. Comparing base (e2a69a1) to head (94c2a74).
Report is 3 commits behind head on master.

❗ Current head 94c2a74 differs from pull request most recent head 58c6be2. Consider uploading reports for the commit 58c6be2 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8909      +/-   ##
==========================================
+ Coverage   82.68%   89.59%   +6.91%     
==========================================
  Files         470      471       +1     
  Lines       30187    30196       +9     
==========================================
+ Hits        24960    27054    +2094     
+ Misses       5227     3142    -2085     

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[elilaird]

Ready for merge/review 👍