Fix Dataset.num_classes in case a transform modifies data.y

[rusty1s]

No description provided.

[codecov]

Codecov Report

Merging #5274 (3459b1c) into master (c875fc9) will decrease coverage by 0.01%.
The diff coverage is 47.05%.

❗ Current head 3459b1c differs from pull request most recent head bfe83c0. Consider uploading reports for the commit bfe83c0 to get more accurate results

@@            Coverage Diff             @@
##           master    #5274      +/-   ##
==========================================
- Coverage   83.15%   83.13%   -0.02%     
==========================================
  Files         337      337              
  Lines       18591    18600       +9     
==========================================
+ Hits        15459    15463       +4     
- Misses       3132     3137       +5     

Impacted Files	Coverage Δ
torch_geometric/data/dataset.py	91.60% <42.85%> (-5.30%)	⬇️
torch_geometric/data/in_memory_dataset.py	84.37% <66.66%> (+3.21%)	⬆️

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