Closes part of #682

[shamikbose]

medical_data updated to bigbio_pairs schema
Passes all tests

• Confirm that this PR is linked to the dataset issue.
• Create the dataloader script biodatasets/my_dataset/my_dataset.py (please use only lowercase and underscore for dataset naming).
• Provide values for the _CITATION, _DATASETNAME, _DESCRIPTION, _HOMEPAGE, _LICENSE, _URLs, _SUPPORTED_TASKS, _SOURCE_VERSION, and _BIGBIO_VERSION variables.
• Implement _info(), _split_generators() and _generate_examples() in dataloader script.
• Make sure that the BUILDER_CONFIGS class attribute is a list with at least one BigBioConfig for the source schema and one for a bigbio schema.
• Confirm dataloader script works with datasets.load_dataset function.
• Confirm that your dataloader script passes the test suite run with python -m tests.test_bigbio biodatasets/my_dataset/my_dataset.py.
• If my dataset is local, I have provided an output of the unit-tests in the PR (please copy paste). This is OPTIONAL for public datasets, as we can test these without access to the data files.

[mariosaenger]

@phlobo Please have a look at this dataset. I refactored the implementation to the new HF-hub-based style. I'm not completely happy with the modeling - essentially it's aspect-based text classification rather than text similarity. However, the former isn't properly supported by any of the BigBio schemes.

[phlobo]

I would pick a different task, but apart from that, the PR looks good!

[phlobo]

@mariosaenger I realized the old version of the dataset is already on the hub: https://huggingface.co/datasets/bigbio/medical_data
should we delete that one?