1、更新requirements以及README.md
2、更新DataLoader
3、更新loss
4、更新model/bert.py内容及适配的测试代码
5、更新reproduction/README.md
6、修复其他测试代码的报错的地方

2. 修改embedding.py中的bug
3. ConllReader默认跳过所有的DOCSTART标签
4. 交换bert的heavy lifting到_bert, 将BertEncoder在bert.py中暴露
5. crf中allow_transition的include_end_start修改为false，以与CRF的默认值适配
6. allow_transition与SpanMetric支持BIOES类型的tag
7. datainfo中增加打印格式化输出

* remove conflicts
* all tests passed

* add code comments
* merge *_saver.py & *_loader.py in io/
* (ancient codes) rename Loss into LossFromTorch

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* remove a lot of out-of-date unit tests, to get testing passed

* rename preprocess.py into utils.py, because nothing about preprocess in it
* anything in loader/ and saver/ is moved directly into io/
* corresponding unit tests are moved to /test/io
* delete fastnlp.py, because we have new and better APIs
* rename Biaffine_parser/run_test.py to Biaffine_parser/main.py; Otherwise, test will fail.
* A looooooooooot of ancient codes to be refined...........