Add directed=False (induced subgraph) support for homogeneous CPU neighbor sampling

[joeykaji]

Addresses #23

Feature Summary

• Implements directed=False (induced subgraph) neighbor sampling for homogeneous graphs on CPU
• Adds three test cases covering: induced subgraph correctness, edge-count difference between directed=True/directed=False, and rejection of unsupported flag combinations

Notes

• Does not yet support disjoint=True or distributed=True in combination with directed=False, but both are explicitly guarded against via TORCH_CHECK
• Heterogeneous graphs are not covered by this PR (still rejects directed=False, unchanged from before)
• For tests, I tried mimicking the other tests on that I saw in test_neighbor.cpp

Happy to adjust based on feedback — this is my first contribution to pyg-lib, so let me know if I've missed any conventions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.66%. Comparing base (bdee3ae) to head (cf57121).
⚠️ Report is 207 commits behind head on master.

❌ Your project check has failed because the head coverage (44.66%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #690       +/-   ##
===========================================
- Coverage   70.13%   44.66%   -25.48%     
===========================================
  Files          37       82       +45     
  Lines        1671     4254     +2583     
  Branches        0      414      +414     
===========================================
+ Hits         1172     1900      +728     
- Misses        499     2352     +1853     
- Partials        0        2        +2     

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