- fix tests

* refine code style
* set up unit tests for Batch, DataSet, FieldArray
* remove a lot of out-of-date unit tests, to get testing passed

- DataSet's __init__ takes a function as argument, rather than class object
- Preprocessor is about to remove. Don't use anymore.
- Remove cross_validate in trainer, because it is rarely used and wired
- Loader.load is expected to be a static method
- Delete sth. in other_modules.py
- Add more tests
- Delete extra sample data

2. changes of names:
  aggregation ----> aggregator
  interaction ----> interactor
  action.py ----> sampler.py
  BasePreprocess ---> Preprocessor
  BaseTester ----> Tester
  BaseTrainer ----> Trainer
3. add more code comments
4. fix bugs in predictor's data_forward
5. in sampler.py, remove Bachifier, fix some codes. but not test
6. remove unused codes in other_modules.py & utils.py
7. update fastnlp.py with new config file names and code comments
8. add data examples in data_for_tests/