Unified dataset format

[zqwerty]

Description:
Unify the format of different datasets so that it is convenient to:

1. evaluate a new model on many different datasets.
2. evaluate existing models on a new dataset.
3. conduct pre-training and domain-transfer experiments.

Example usage:

from convlab2.util import load_dataset, load_ontology, load_database

dataset = load_dataset('multiwoz21')
ontology = load_ontology('multiwoz21')
database = load_database('multiwoz21')