Expose dataset properties

[cthoyt]

Summary

It wasn't clear where the magic parameter numbers came from in the existing two examples - it turns out that a few of the inputs to a given model need to come from the dataset. This PR standardizes the names of those parameters and makes them easier to get from an instance of a dataset by exposing them as python properties

• [x] Code passes all tests
• [x] Unit tests provided for these changes
• [x] Documentation and docstrings added for these changes
• [x] Figure out where the number 69 for EPGCN-DS + DrugCombDB comes from (update: a constant from inside torchdrug)

Changes

• Add lru_cache to functions that get datasets' parts
• Expose several properties for a dataset including # drugs, # features for each drug, # contexts, # features for each context, and # triples.
• Rename parameters to models corresponding to drug_channels and context_channels to correspond respectively to # features for each drug and # features for each context.
• Removed default values for drug_channels and context_channels (and update the test that checks for default values in each model's def __init__()
• Use setUpClass to reduce unnecessary reloading of datasets in testing
• Update examples for EPGCN-DS and DeepSynergy to reflect changes

[cthoyt]

Alright the issue with the torchdrug widths was solved by adding a constant.

[codecov-commenter]

Codecov Report

Merging #52 (2292b4a) into main (8fdd199) will decrease coverage by 0.60%. The diff coverage is 88.63%.

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
- Coverage   98.45%   97.84%   -0.61%     
==========================================
  Files          27       28       +1     
  Lines         712      744      +32     
==========================================
+ Hits          701      728      +27     
- Misses         11       16       +5     

Impacted Files	Coverage Δ
chemicalx/models/__init__.py	100.00% <ø> (ø)
chemicalx/models/deepsynergy.py	100.00% <ø> (ø)
chemicalx/data/labeledtriples.py	97.87% <50.00%> (-2.13%)	:arrow_down:
chemicalx/data/datasetloader.py	90.78% <81.81%> (-3.95%)	:arrow_down:
chemicalx/constants.py	100.00% <100.00%> (ø)
chemicalx/models/epgcnds.py	100.00% <100.00%> (ø)
tests/unit/test_models.py	100.00% <100.00%> (ø)

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