[DOC][CI] update to use mxnet 1.4.1

[szha]

Description

update doc and CI to use mxnet 1.4.1

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

• update doc and CI to use mxnet 1.4.1

[codecov]

Codecov Report

Merging #771 into master will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #771      +/-   ##
==========================================
+ Coverage   90.63%   90.72%   +0.09%     
==========================================
  Files          64       64              
  Lines        6233     6233              
==========================================
+ Hits         5649     5655       +6     
+ Misses        584      578       -6

Impacted Files	Coverage Δ
src/gluonnlp/data/dataloader.py	88.79% <0%> (+5.17%)	⬆️

[codecov]

Codecov Report

❗ No coverage uploaded for pull request head (mx_version@f2d2e86). Click here to learn what that means.
The diff coverage is n/a.

[mli]

Job PR-771/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-771/1/index.html