Merge branch 'unified_dataset' into pre-training

convlab2/util/unified_datasets_util.py

data/unified_datasets/README.md

data/unified_datasets/camrest/shuffled_dial_ids.json

data/unified_datasets/check.py

data/unified_datasets/metalwoz/shuffled_dial_ids.json

data/unified_datasets/multiwoz21/shuffled_dial_ids.json

data/unified_datasets/sgd/shuffled_dial_ids.json

data/unified_datasets/tm1/shuffled_dial_ids.json

data/unified_datasets/tm2/shuffled_dial_ids.json

data/unified_datasets/tm3/shuffled_dial_ids.json