feat: initialize SPMD-based edge-partitioned message passing for GFM-RAG

[rajveer43]

This PR introduces SPMD-style distributed message passing with edge-level partitioning for the GFM-RAG model to enable reasoning over large-scale graphs.

Closes #10347

[codecov]

Codecov Report

❌ Patch coverage is 0% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.03%. Comparing base (c211214) to head (5942608).
⚠️ Report is 107 commits behind head on master.

Files with missing lines	Patch %	Lines
torch_geometric/nn/conv/spdm_gcn_conv.py	0.00%	38 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10451      +/-   ##
==========================================
- Coverage   86.11%   85.03%   -1.09%     
==========================================
  Files         496      511      +15     
  Lines       33655    35992    +2337     
==========================================
+ Hits        28981    30604    +1623     
- Misses       4674     5388     +714     

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