'monailabel datasets --download' always fails the hash check on MONAI >= 1.6 #1920

Problem

monailabel datasets --download --name Task04_Hippocampus (and every other Decathlon dataset) fails with MONAI >= 1.6:

monai.apps.utils.HashCheckError: sha256 hash check of downloaded file failed:
url='https://msd-for-monai.s3-us-west-2.amazonaws.com/Task04_Hippocampus.tar', ...
expected sha256=9d24dba78a72977dbd1d2e110310f31b

monailabel/main.py passes DecathlonDataset.md5[name] (an md5 digest) to download_and_extract without saying which algorithm it is:

download_and_extract(resource[args.name], tarfile_name, root_dir, md5.get(args.name))

MONAI 1.6 changed the default hash_type of download_url / download_and_extract from md5 to sha256, so the md5 digest is now compared as a sha256 and never matches. This is what makes tests/unit/test_main.py::test_datasets_download fail.

Fix

Pass hash_type="md5" explicitly. This works on both old and new MONAI.