Add copyright header to unit tests

[gpengzhi]

No description provided.

[codecov]

Codecov Report

Merging #287 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
+ Coverage   82.56%   82.62%   +0.06%     
==========================================
  Files         207      207              
  Lines       15941    16003      +62     
==========================================
+ Hits        13161    13223      +62     
  Misses       2780     2780

Impacted Files	Coverage Δ
texar/torch/utils/utils_test.py	98.93% <ø> (+0.01%)	⬆️
texar/torch/modules/encoders/bert_encoder_test.py	54.54% <ø> (+0.69%)	⬆️
...xar/torch/modules/decoders/decoder_helpers_test.py	95.65% <ø> (+0.19%)	⬆️
texar/torch/data/embedding_test.py	95.55% <ø> (ø)	⬆️
...xar/torch/modules/encoders/roberta_encoder_test.py	54.54% <ø> (+0.69%)	⬆️
texar/torch/losses/entropy_test.py	97.87% <ø> (+0.04%)	⬆️
texar/torch/modules/encoders/rnn_encoders_test.py	98.87% <ø> (ø)	⬆️
texar/torch/modules/decoders/rnn_decoders_test.py	99.31% <ø> (ø)	⬆️
...ch/data/tokenizers/sentencepiece_tokenizer_test.py	99% <ø> (+0.01%)	⬆️
texar/torch/modules/encoders/xlnet_encoder_test.py	52.38% <ø> (+0.76%)	⬆️
... and 64 more

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