[BUGFIX] Fix multiprocessing bug in create_pretraining_data.py for bert

[hankcs]

Description

Fix multiprocessing bug which causes unnecessary dispatching to single worker.

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

• Only consider nworker >= 2 as the condition for multiprocessing.

Comments

I guess the other condition branch is intended for serial execution, while it will never be triggered when nworker is 1.

[mli]

Job PR-649/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-649/1/index.html

[codecov]

Codecov Report

Merging #649 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #649      +/-   ##
==========================================
+ Coverage   63.76%   63.77%   +<.01%     
==========================================
  Files         141      141              
  Lines       12870    12870              
==========================================
+ Hits         8207     8208       +1     
+ Misses       4663     4662       -1

Flag	Coverage Δ
#PR435	66.95% <ø> (?)
#PR466	67.03% <ø> (?)
#PR588	67.03% <ø> (?)
#PR612	67.03% <ø> (?)
#PR639	66.93% <ø> (?)
#PR640	66.93% <ø> (?)
#PR647	?
#PR648	67.03% <ø> (?)
#PR649	67.03% <ø> (?)
#master	63.77% <ø> (ø)	⬆️
#notserial	42.67% <ø> (+0.03%)	⬆️
#py2	63.53% <ø> (ø)	⬆️
#py3	63.65% <ø> (+0.04%)	⬆️
#serial	49.18% <ø> (-0.04%)	⬇️

[codecov]

Codecov Report

Merging #649 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #649      +/-   ##
==========================================
- Coverage   63.76%   63.72%   -0.04%     
==========================================
  Files         141      141              
  Lines       12870    12870              
==========================================
- Hits         8207     8202       -5     
- Misses       4663     4668       +5

Flag	Coverage Δ
#PR649	63.72% <0%> (?)
#master	?
#notserial	42.62% <0%> (-0.02%)	⬇️
#py2	63.48% <0%> (-0.04%)	⬇️
#py3	63.61% <0%> (ø)	⬆️
#serial	49.18% <0%> (-0.04%)	⬇️

[mli]

Job PR-649/3 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-649/3/index.html