change dataset behaviour when dataset is overridden

[Padarn]

PR to address #4567

Changes in the behavior of how 'download' and 'process' are resolved by Dataset. Prior to this change, the functions needed to be defined in the child class, after this change they only need to be implemented in some class that is not the base Dataset.

[Padarn]

Hey @rusty1s what is your intended interaction with CHANGELOG.md? Do you want all merges to master to add one, or only larger changes?

[rusty1s]

Every PR should at least change the CHANGELOG.md for now (either by writing a new line or adding the PR link to a larger feature).

[codecov]

Codecov Report

Merging #4586 (f0ff396) into master (be97c5b) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4586      +/-   ##
==========================================
- Coverage   82.90%   82.78%   -0.12%     
==========================================
  Files         315      315              
  Lines       16592    16588       -4     
==========================================
- Hits        13756    13733      -23     
- Misses       2836     2855      +19     

Impacted Files	Coverage Δ
torch_geometric/data/in_memory_dataset.py	77.27% <ø> (-0.67%)	⬇️
torch_geometric/data/dataset.py	96.80% <100.00%> (-0.80%)	⬇️
torch_geometric/nn/conv/utils/typing.py	81.25% <0.00%> (-17.50%)	⬇️
torch_geometric/io/tu.py	93.58% <0.00%> (-2.57%)	⬇️
torch_geometric/nn/models/mlp.py	98.41% <0.00%> (-1.59%)	⬇️
torch_geometric/transforms/gdc.py	78.17% <0.00%> (-1.02%)	⬇️
torch_geometric/nn/conv/rgat_conv.py	83.76% <0.00%> (-0.53%)	⬇️
torch_geometric/graphgym/utils/comp_budget.py	15.51% <0.00%> (+0.51%)	⬆️

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