Warn on accessing dataset.data

[rusty1s]

No description provided.

[codecov]

Codecov Report

Merging #6318 (938b9f0) into master (afecff0) will increase coverage by 0.00%.
The diff coverage is 82.35%.

@@           Coverage Diff           @@
##           master    #6318   +/-   ##
=======================================
  Coverage   84.44%   84.44%           
=======================================
  Files         385      385           
  Lines       21368    21374    +6     
=======================================
+ Hits        18044    18050    +6     
  Misses       3324     3324           

Impacted Files	Coverage Δ
torch_geometric/graphgym/loader.py	40.25% <57.14%> (ø)
torch_geometric/data/in_memory_dataset.py	85.50% <100.00%> (+1.38%)	⬆️

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