Improve the readability of the training script.

[liuzh47]

Description

(Brief description on what this PR is about)
[MAINTENANCE]This fix replaces magic numbers with the name

Checklist

Essentials

• [X ] PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• [ X] Changes are complete (i.e. I finished coding on this PR)
• [ X] All changes have test coverage
• [ X] Code is well-documented

cc @dmlc/gluon-nlp-team

[codecov]

Codecov Report

Merging #1006 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1006   +/-   ##
=======================================
  Coverage   88.27%   88.27%           
=======================================
  Files          67       67           
  Lines        6254     6254           
=======================================
  Hits         5521     5521           
  Misses        733      733

[mli]

Job PR-1006/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1006/1/index.html

[mli]

Job PR-1006/2 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1006/2/index.html