Only add true negatives in add_random_edge augmentation

[rusty1s]

Fixes #7653

[codecov]

Codecov Report

Merging #7654 (c791945) into master (2ec1c4b) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7654      +/-   ##
==========================================
- Coverage   91.60%   91.60%   -0.01%     
==========================================
  Files         452      452              
  Lines       25481    25469      -12     
==========================================
- Hits        23342    23330      -12     
  Misses       2139     2139              

Impacted Files	Coverage Δ
torch_geometric/utils/__init__.py	100.00% <100.00%> (ø)
torch_geometric/utils/augmentation.py	100.00% <100.00%> (ø)
torch_geometric/utils/negative_sampling.py	98.54% <100.00%> (ø)

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