add soloist e2e model

convlab2/e2e/soloist/multiwoz/config.py

...lab2/e2e/soloist/multiwoz/db/attraction_db_processed.json

convlab2/e2e/soloist/multiwoz/db/hospital_db_processed.json

convlab2/e2e/soloist/multiwoz/db/hotel_db_processed.json

convlab2/e2e/soloist/multiwoz/db/ontology.json

convlab2/e2e/soloist/multiwoz/db/police_db_processed.json

...lab2/e2e/soloist/multiwoz/db/restaurant_db_processed.json

convlab2/e2e/soloist/multiwoz/db/taxi_db_processed.json

convlab2/e2e/soloist/multiwoz/db/train_db_processed.json

convlab2/e2e/soloist/multiwoz/db/value_set_processed.json

convlab2/e2e/soloist/multiwoz/db_ops.py

convlab2/e2e/soloist/multiwoz/multiwoz_dataset.py

convlab2/e2e/soloist/multiwoz/script/create_mwoz_e2e_json.py

convlab2/e2e/soloist/multiwoz/soloist.py

convlab2/e2e/soloist/multiwoz/soloist_net.py

convlab2/e2e/soloist/multiwoz/utils.py