Add sequence_tagging example

[gpengzhi]

Adapted from sequence_tagging in texar-tf.

[codecov[bot]]

Codecov Report

Merging #302 into master will not change coverage by %. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #302   +/-   ##
=======================================
  Coverage   79.76%   79.76%           
=======================================
  Files         133      133           
  Lines       11122    11122           
=======================================
  Hits         8872     8872           
  Misses       2250     2250           

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