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

Attention: Patch coverage is 97.72727% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.85%. Comparing base (dafbd30) to head (c48cbe9).
Report is 12 commits behind head on master.

❗ Current head c48cbe9 differs from pull request most recent head f88f0d3

Please upload reports for the commit f88f0d3 to get more accurate results.

Files	Patch %	Lines
torch_geometric/typing.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8909      +/-   ##
==========================================
- Coverage   88.25%   86.85%   -1.41%     
==========================================
  Files         473      463      -10     
  Lines       30844    30569     -275     
==========================================
- Hits        27221    26550     -671     
- Misses       3623     4019     +396     

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[elilaird]

Ready for merge/review 👍