Fix docs issues

[gpengzhi]

Fix CI error in #260

[codecov]

Codecov Report

Merging #290 into master will increase coverage by 0.39%.
The diff coverage is 70.23%.

@@            Coverage Diff             @@
##           master     #290      +/-   ##
==========================================
+ Coverage   82.25%   82.64%   +0.39%     
==========================================
  Files         206      207       +1     
  Lines       15928    16005      +77     
==========================================
+ Hits        13102    13228     +126     
+ Misses       2826     2777      -49

Impacted Files	Coverage Δ
texar/torch/modules/encoders/bert_encoder_test.py	54.54% <ø> (ø)	⬆️
...xar/torch/modules/decoders/decoder_helpers_test.py	95.65% <ø> (ø)	⬆️
...xar/torch/modules/encoders/roberta_encoder_test.py	54.54% <ø> (ø)	⬆️
texar/torch/losses/entropy_test.py	97.87% <ø> (ø)	⬆️
texar/torch/losses/adv_losses_test.py	94.11% <ø> (ø)	⬆️
texar/torch/modules/encoders/xlnet_encoder_test.py	52.38% <ø> (ø)	⬆️
...torch/modules/classifiers/conv_classifiers_test.py	96.55% <ø> (ø)	⬆️
texar/torch/modules/embedders/embedder_base.py	94.11% <ø> (ø)	⬆️
texar/torch/losses/pg_losses_test.py	98.03% <ø> (ø)	⬆️
...orch/modules/decoders/transformer_decoders_test.py	98.76% <ø> (ø)	⬆️
... and 196 more

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