[Feature] Enable support for GNNs from torch_geometric.nn.attention

[Xmaster6y]

Allows other GNN types from torch_geometric.nn.attention.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.13%. Comparing base (cbe5c58) to head (a2310c5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
benchmarl/models/gnn.py	63.63%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #227      +/-   ##
==========================================
- Coverage   78.55%   78.13%   -0.42%     
==========================================
  Files          85       85              
  Lines        4448     4451       +3     
==========================================
- Hits         3494     3478      -16     
- Misses        954      973      +19     

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