* rename callback methods. Use fastai's notation.
* add a new callback method - on_valid_begin

* move used readers from reproduction to io/dataset_loader.py
(API shall not call anything from reproduction/)

- refine & fix Transformer Encoder
- refine & speed up biaffine parser

add parser API

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

* 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...........