[DOC] add the basic documentation for the embedding API

[acphile]

Description

Add the basic documentation for the embedding API

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[bot]]

Codecov Report

Merging #1281 into numpy will increase coverage by 1.49%. The diff coverage is n/a.

@@            Coverage Diff             @@
##            numpy    #1281      +/-   ##
==========================================
+ Coverage   82.65%   84.14%   +1.49%     
==========================================
  Files          41       41              
  Lines        5702     6157     +455     
==========================================
+ Hits         4713     5181     +468     
+ Misses        989      976      -13     

Impacted Files	Coverage Δ
src/gluonnlp/embedding/embed_loader.py	81.52% <ø> (ø)
src/gluonnlp/utils/config.py	61.53% <0.00%> (-15.39%)	:arrow_down:
src/gluonnlp/utils/testing.py	94.11% <0.00%> (-3.03%)	:arrow_down:
src/gluonnlp/utils/misc.py	48.18% <0.00%> (-1.45%)	:arrow_down:
src/gluonnlp/models/albert.py	95.47% <0.00%> (-1.22%)	:arrow_down:
src/gluonnlp/attention_cell.py	79.52% <0.00%> (-0.22%)	:arrow_down:
src/gluonnlp/models/transformer_xl.py	82.52% <0.00%> (-0.20%)	:arrow_down:
src/gluonnlp/data/tokenizers.py	77.69% <0.00%> (-0.03%)	:arrow_down:
src/gluonnlp/layers.py	86.78% <0.00%> (ø)
src/gluonnlp/models/xlmr.py	88.23% <0.00%> (+1.35%)	:arrow_up:
... and 5 more

[leezu]

Thank you @acphile. Please still add the Makefile with the docs_local target so that the notebook compilation step can be executed. For now I built the docs via the following steps, but it doesn't compile the notebook of course:

pip install sphinx_gallery sphinx_autodoc_typehints nbsphinx mxtheme
cd docs
make html