- add DataSet, Instance, Field to represent data in different levels
- encapsulate batching method in Batch class
- modify samplers in action.py to fit Batch
- preprocessor.run returns DataSet, instead of list
- Use Batch in Trainer/Tester
- add required_arg "task" in Trainer/Tester
- remove SeqLabelTrainer/SeqLabelTester dependencies successfully. They empty classes to deprecate.
- modify SeqLabeling model, add another argument in forward, in order to compute mask inside model
- test\model\seq_labeling.py works