[Script] Minor enhancement for bert pre-training script

[eric-haibin-lin]

Description

Add support for custom optimizer. Fix checkpoint interval check.

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

[codecov]

Codecov Report

Merging #1121 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1121   +/-   ##
=======================================
  Coverage   88.34%   88.34%           
=======================================
  Files          66       66           
  Lines        6290     6290           
=======================================
  Hits         5557     5557           
  Misses        733      733

[mli]

Job PR-1121/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1121/1/index.html

[leezu]

LGTM