feat/gfn-unpooling

[Oisin-M]

This PR proposes adding the GFNUnpooling layer to PyG. I have motivated why I think this would be worth adding in the linked issue.
Close #10587

Checklist:

• Adds new nn.unpool.GFNUnpooling layer
• Adds test
• Updates docs

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.29%. Comparing base (c211214) to head (367d1f9).
⚠️ Report is 156 commits behind head on master.

Files with missing lines	Patch %	Lines
torch_geometric/nn/unpool/gfn.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10588      +/-   ##
==========================================
- Coverage   86.11%   85.29%   -0.82%     
==========================================
  Files         496      511      +15     
  Lines       33655    35995    +2340     
==========================================
+ Hits        28981    30701    +1720     
- Misses       4674     5294     +620     

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