Merge New SetSUMBT Code

convlab/dst/evaluate_unified_datasets.py

convlab/dst/setsumbt/configs/setsumbt_multitask.json

convlab/dst/setsumbt/configs/setsumbt_multiwoz21.json

convlab/dst/setsumbt/configs/setsumbt_pretrain.json

convlab/dst/setsumbt/configs/setsumbt_sgd.json

convlab/dst/setsumbt/configs/setsumbt_tm.json

convlab/dst/setsumbt/dataset/unified_format.py

convlab/dst/setsumbt/dataset/utils.py

convlab/dst/setsumbt/do/nbt.py

convlab/dst/setsumbt/get_golden_labels.py

convlab/dst/setsumbt/modeling/training.py

convlab/dst/setsumbt/utils.py