[Model] GNN

[matteobettini]

This PR aims at introducing a GNN model customizable via torch_geometric.

This is currently on hold because of:

pytorch/rl#1576

[codecov-commenter]

Codecov Report

Attention: 63 lines in your changes are missing coverage. Please review.

Comparison is base (c6114e0) 89.47% compared to head (935e49e) 45.26%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
benchmarl/models/gnn.py	34.56%	53 Missing ⚠️
test/test_pettingzoo.py	62.50%	3 Missing ⚠️
test/test_smacv2.py	57.14%	3 Missing ⚠️
test/test_vmas.py	57.14%	3 Missing ⚠️
test/conftest.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #30       +/-   ##
===========================================
- Coverage   89.47%   45.26%   -44.21%     
===========================================
  Files          54       56        +2     
  Lines        2451     2556      +105     
===========================================
- Hits         2193     1157     -1036     
- Misses        258     1399     +1141     

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[matteobettini]

@janblumenkamp @Acciorocketships here is the GNN!