[Fix] Some minor fixes for AMLC Tutorial

[sxjscience]

Description

Some minor doc + API improvements.

Checklist

Essentials

• [x] PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• [x] Changes are complete (i.e. I finished coding on this PR)
• [x] All changes have test coverage
• [x] 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[bot]]

Codecov Report

Merging #1355 into master will increase coverage by 10.91%. The diff coverage is 80.00%.

@@             Coverage Diff             @@
##           master    #1355       +/-   ##
===========================================
+ Coverage   70.70%   81.61%   +10.91%     
===========================================
  Files         105       52       -53     
  Lines       12445     6882     -5563     
===========================================
- Hits         8799     5617     -3182     
+ Misses       3646     1265     -2381     

Impacted Files	Coverage Δ
src/gluonnlp/models/bart.py	70.62% <ø> (-15.63%)	:arrow_down:
src/gluonnlp/models/transformer.py	98.51% <ø> (-0.43%)	:arrow_down:
src/gluonnlp/utils/preprocessing.py	100.00% <ø> (ø)
src/gluonnlp/utils/parameter.py	86.53% <37.50%> (-8.92%)	:arrow_down:
src/gluonnlp/models/electra.py	57.89% <50.00%> (-18.56%)	:arrow_down:
src/gluonnlp/utils/misc.py	51.08% <85.71%> (-6.83%)	:arrow_down:
src/gluonnlp/layers.py	86.93% <100.00%> (+0.15%)	:arrow_up:
src/gluonnlp/models/__init__.py	67.74% <100.00%> (-28.93%)	:arrow_down:
src/gluonnlp/models/albert.py	87.80% <100.00%> (-7.67%)	:arrow_down:
src/gluonnlp/models/bert.py	87.67% <100.00%> (-7.20%)	:arrow_down:
... and 69 more

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