add dataloader for ptb_lm

[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

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
2 out of 11 committers have signed the CLA.

:white_check_mark: jerrywgz
:white_check_mark: LielinJiang
:x: parap1uie-s
:x: heavengate
:x: frankwhzhang
:x: xyzhou-puck
:x: FrostML
:x: cjt222
:x: LDOUBLEV
:x: xjqbest
:x: JepsonWong
_{You have signed the CLA already but the status is still pending? Let us recheck it.}