[DOC] avoid using bash in BERT tutorial

[eric-haibin-lin]

Description

http://ci.mxnet.io/blue/organizations/jenkins/GluonNLP-py3-master-gpu-doc/detail/PR-648/1/pipeline shows that the BERT tutorial cannot run with the bash command. It's still not clear why it happens to me. This PR tries to replace bash with python code.

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

❗ No coverage uploaded for pull request head (fix-tutorial@e792529). Click here to learn what that means.
The diff coverage is n/a.

[codecov]

Codecov Report

Merging #698 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #698   +/-   ##
=======================================
  Coverage   90.94%   90.94%           
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  Files          64       64           
  Lines        5887     5887           
=======================================
  Hits         5354     5354           
  Misses        533      533

[mli]

Job PR-698/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-698/1/index.html