Description

Fixes an issue with MedNISTDataset where choosing a different random seed returns a differently sized dataset.
Reproduce by running

data0 = monai.apps.datasets.MedNISTDataset(root_dir="./tmp", transform=[], section="test", download=False, seed=0)
data42 = monai.apps.datasets.MedNISTDataset(root_dir="./tmp", transform=[], section="test", download=False, seed=42)
assert len(data0) != len(data42)

Instead of seeing whether a chosen random value falls between the train/val/test split serially, this PR uses the same kind of implementation for choosing random indices all at once like how it's done in DecathlonDataset.

Also modified the existing MedNIST test to use the full dataset length for comparison and to test dataset lengths for different seeds.

It seems like https://github.com/Project-MONAI/tutorials/blob/master/2d_classification/mednist_tutorial.ipynb may also need these changes.

Status

Ready

Types of changes

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