fix prepare_openwebtext

[ZiyueHuang]

Description

The current implementation will raise TypeError: 'NoneType' object is not iterable, since random.shuffle() shuffles the input in place and returns None.

cc @szhengac @sxjscience

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

cc @dmlc/gluon-nlp-team

[codecov]

Codecov Report

Merging #1289 into numpy will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            numpy    #1289   +/-   ##
=======================================
  Coverage   83.60%   83.60%           
=======================================
  Files          42       42           
  Lines        6375     6375           
=======================================
  Hits         5330     5330           
  Misses       1045     1045           

[zheyuye]

Thanks for fixing this.