Fix GAT with out_channels != None

[rusty1s]

Fixes #4298

[codecov]

Codecov Report

Merging #4299 (ae522e9) into master (0833e05) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4299   +/-   ##
=======================================
  Coverage   82.69%   82.70%           
=======================================
  Files         309      309           
  Lines       15861    15864    +3     
=======================================
+ Hits        13117    13120    +3     
  Misses       2744     2744           

Impacted Files	Coverage Δ
torch_geometric/nn/models/basic_gnn.py	90.00% <100.00%> (+0.30%)	⬆️

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