enhance mednist dataset

[rijobro]

Adds get_num_classes method to MedNISTDataset, and also stores the text associated with each label.

e.g.,

print(train_ds.get_num_classes())
print(train_ds[0]["class_name"])

Status

Ready

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh --codeformat --coverage.
• Quick tests passed locally by running ./runtests.sh --quick.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[wyli]

thanks!