* update SamplerHelper.batch

* update batch_size, delete a useless log

* update SamplerHelper.batch

* update SamplerHelper.batch, change cmp_fn arg to key

* Update TranslationDataset and IWSLT

* fix doc error

* update lrscheduler

* minor fix

* add pre-commit

* minor fix

* Add __len__ to squad dataset

* minor fix

* Add dureader robust prototype

* dataset implement

* minor fix

* fix var name

* add dureader-yesno train script and dataset

* add readme and fix md5sum

* integrete dureader datasets

* change var names: segment to mode, root to data_file

* minor fix

* update var name