[DOC] Add missing docs for nlp.utils

[eric-haibin-lin]

Description

The website API doc does not include nlp.utils APIs.

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 #936 into master will decrease coverage by 13.69%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #936      +/-   ##
==========================================
- Coverage   89.95%   76.25%   -13.7%     
==========================================
  Files          67       67              
  Lines        6360     6360              
==========================================
- Hits         5721     4850     -871     
- Misses        639     1510     +871

Impacted Files	Coverage Δ
src/gluonnlp/data/batchify/embedding.py	17.32% <0%> (-80.32%)	⬇️
src/gluonnlp/data/candidate_sampler.py	19.23% <0%> (-73.08%)	⬇️
src/gluonnlp/optimizer/lamb.py	23.8% <0%> (-66.67%)	⬇️
src/gluonnlp/utils/version.py	33.33% <0%> (-66.67%)	⬇️
src/gluonnlp/utils/parallel.py	34.04% <0%> (-63.83%)	⬇️
src/gluonnlp/optimizer/bert_adam.py	27.45% <0%> (-58.83%)	⬇️
src/gluonnlp/utils/parameter.py	23.72% <0%> (-57.63%)	⬇️
src/gluonnlp/loss/activation_regularizer.py	35.48% <0%> (-51.62%)	⬇️
...p/data/corpora/large_text_compression_benchmark.py	40.74% <0%> (-50%)	⬇️
src/gluonnlp/loss/loss.py	50% <0%> (-43.75%)	⬇️
... and 24 more

[mli]

Job PR-936/3 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-936/3/index.html

[szha]

We will need to improve the code coverage of the utils module.

[eric-haibin-lin]

I'm not sure why this PR affects the code coverage.. Every API under nlp.utils are tested in https://github.com/dmlc/gluon-nlp/blob/master/tests/unittest/test_utils.py