Fix is_edge_attr in case num_edges == 1

[rusty1s]

No description provided.

[codecov-commenter]

Codecov Report

Merging #3880 (9579dca) into master (965f1f8) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3880      +/-   ##
==========================================
- Coverage   81.77%   81.68%   -0.10%     
==========================================
  Files         299      299              
  Lines       15158    15202      +44     
==========================================
+ Hits        12396    12417      +21     
- Misses       2762     2785      +23     

Impacted Files	Coverage Δ
torch_geometric/data/storage.py	81.16% <100.00%> (-0.13%)	⬇️
torch_geometric/data/temporal.py	38.09% <0.00%> (+6.84%)	⬆️

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