[SCRIPTS]update pre-trained model link for transformer

[ciyongch]

Description

This PR is to update the default transformer pre-trained model to the link of e25287c5 to align with website.

Checklist

Essentials

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

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

@leezu @eric-haibin-lin

[codecov]

Codecov Report

Merging #1117 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1117   +/-   ##
=======================================
  Coverage   88.22%   88.22%           
=======================================
  Files          67       67           
  Lines        6312     6312           
=======================================
  Hits         5569     5569           
  Misses        743      743

[mli]

Job PR-1117/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1117/1/index.html