[new] 兼容pytorch的DataLoader，替换Batch为DataSetIter

[choosewhatulike]

Description：兼容pytorch的DataLoader，替换Batch为DataSetIter

• [new] DataSetIter, TorchLoaderIter, BatchIter
• [remove] Batch
• [change] Trainer 的API

Main reason: 使Trainer更快也更易使用

@修改过这个文件的人
@核心开发人员

[codecov-io]

Codecov Report

Merging #166 into dev0.5.0 will increase coverage by 0.27%.
The diff coverage is 87.11%.

@@             Coverage Diff              @@
##           dev0.5.0     #166      +/-   ##
============================================
+ Coverage     64.68%   64.96%   +0.27%     
============================================
  Files            88       88              
  Lines          9083     9107      +24     
============================================
+ Hits           5875     5916      +41     
+ Misses         3208     3191      -17

Impacted Files	Coverage Δ
fastNLP/__init__.py	100% <ø> (ø)	⬆️
test/models/model_runner.py	98.75% <100%> (ø)	⬆️
fastNLP/core/__init__.py	100% <100%> (ø)	⬆️
test/core/test_trainer.py	95.71% <100%> (ø)	⬆️
test/core/test_callbacks.py	100% <100%> (ø)	⬆️
test/core/test_batch.py	100% <100%> (ø)	⬆️
fastNLP/core/predictor.py	73.17% <100%> (ø)	⬆️
test/test_tutorials.py	100% <100%> (ø)	⬆️
fastNLP/modules/encoder/embedding.py	21.33% <50%> (ø)	⬆️
fastNLP/core/tester.py	85.88% <62.5%> (-2.87%)	⬇️
... and 4 more

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