language_model_ptb example

[gpengzhi]

resolve #232 (including distributed_gpu and language_model_ptb examples)

[codecov]

Codecov Report

Merging #281 into master will decrease coverage by 0.01%.
The diff coverage is 59.52%.

@@            Coverage Diff             @@
##           master     #281      +/-   ##
==========================================
- Coverage   83.07%   83.05%   -0.02%     
==========================================
  Files         195      195              
  Lines       15324    15324              
==========================================
- Hits        12730    12728       -2     
- Misses       2594     2596       +2

Impacted Files	Coverage Δ
texar/torch/modules/embedders/embedder_utils.py	100% <ø> (ø)	⬆️
...exar/torch/modules/embedders/position_embedders.py	90.43% <ø> (ø)	⬆️
texar/torch/modules/embedders/embedders.py	91.66% <ø> (ø)	⬆️
texar/torch/modules/embedders/embedder_base.py	94.11% <ø> (ø)	⬆️
...exar/torch/data/tokenizers/xlnet_tokenizer_test.py	96.85% <ø> (ø)	⬆️
texar/torch/data/tokenizers/xlnet_tokenizer.py	85.38% <0%> (ø)	⬆️
texar/torch/modules/embedders/embedders_test.py	98.11% <100%> (ø)	⬆️
texar/torch/data/data_utils.py	42.38% <14.28%> (ø)	⬆️
texar/torch/modules/pretrained/pretrained_base.py	50% <14.28%> (-0.49%)	⬇️
texar/torch/core/optimization.py	87.23% <75%> (-1.34%)	⬇️
... and 7 more

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