[CI] Enable integration tests on CPU

[leezu]

Description

See #944. CPU integration tests were previously not executed.

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

cc @dmlc/gluon-nlp-team

[codecov]

Codecov Report

Merging #957 into master will increase coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #957      +/-   ##
==========================================
+ Coverage   90.13%   90.35%   +0.21%     
==========================================
  Files          67       67              
  Lines        6438     6438              
==========================================
+ Hits         5803     5817      +14     
+ Misses        635      621      -14

Impacted Files	Coverage Δ
src/gluonnlp/utils/files.py	49.01% <0%> (+3.92%)	⬆️
src/gluonnlp/model/bert.py	90.26% <0%> (+5.3%)	⬆️

[mli]

Job PR-957/2 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-957/2/index.html

[mli]

Job PR-957/3 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-957/3/index.html

[mli]

Job PR-957/4 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-957/4/index.html