- fix tests

- fix bugs

- add csv, conll, json filereader
- update dataloader
- remove useless dataloader
- fix trainer loss print
- fix tests

2.Vocabulary.from_dataset(), index_dataset()提供报错时的vocab顺序
3.embedloader在embed读取时遇到不规则的数据跳过这一行.

2. vocabulary增加from_dataset(), index_dataset()函数。避免需要多行写index dataset的问题。
3. 在utils中新增一个cache_result()修饰器，用于cache函数的返回值。
4. callback中新增update_every属性

* refine code style

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

* 更新Loss的接口形参跟metric保持一致
* 添加对几种loss的测试
* embed_loader采用维度独立的方法采样
* 对应测试代码的修改

* 更新测试代码，跑通所有测试，覆盖率65%
* refine代码规范和某些注释
* fix tester self.use_cuda未赋值先使用的bug
* 添加tutorial样例数据——tutorial_sample_dataset.csv
* 【unsolved】embed_loader在计算np.cov时遇到segmentation fault

* dataset检查slice开始位置，确保结果不为空
* fieldarray检查content不为空
* optimizer接受的model params是一个generator，不能赋值
* code style refine

* 添加fast_load_embedding方法，用vocab的词索引pre-trained中的embedding
* 如果vocab有词没出现在pre-train中，从已有embedding中正态采样

Update embed_loader:
* add fast_load_embedding method, to index pre-trained embedding with words in Vocab
* If words in Vocab are not exist in pre-trained, sample them from normal distribution computed by current embeddings

* 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* remove a lot of out-of-date unit tests, to get testing passed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...........