Fix GMMConv when edge_attr is None

[xnuohz]

close #10143

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.91%. Comparing base (c211214) to head (4cb91e9).
⚠️ Report is 179 commits behind head on master.

Files with missing lines	Patch %	Lines
torch_geometric/nn/conv/gmm_conv.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10182      +/-   ##
==========================================
- Coverage   86.11%   85.91%   -0.20%     
==========================================
  Files         496      510      +14     
  Lines       33655    36018    +2363     
==========================================
+ Hits        28981    30946    +1965     
- Misses       4674     5072     +398     

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