Created by: JepsonWong
add dataloader for ptb_lm.
1.7.0:
time cost 165.94562101364136
-- Epoch:[0]; ppl: 266.13202
time cost 111.05947875976562
-- Epoch:[1]; ppl: 133.05743
time cost 111.44368863105774
-- Epoch:[2]; ppl: 102.36475
time cost 129.24074625968933
-- Epoch:[3]; ppl: 85.98092
time cost 128.98609447479248
-- Epoch:[4]; ppl: 65.60556
develop:
time cost 119.87573456764221
-- Epoch:[0]; ppl: 266.54718
time cost 99.7285053730011
-- Epoch:[1]; ppl: 132.06448
time cost 100.61404538154602
-- Epoch:[2]; ppl: 101.89635
time cost 102.5845696926117
-- Epoch:[3]; ppl: 86.17884
time cost 101.57794308662415
-- Epoch:[4]; ppl: 65.99828