[DATASET] add atis and snips

[szha]

Description

add atis and snips

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

• add ATIS and Snips datasets for intent classification and slot labeling

[codecov]

Codecov Report

❗ No coverage uploaded for pull request head (icsl@9713233). Click here to learn what that means.
The diff coverage is n/a.

[codecov]

Codecov Report

Merging #816 into master will decrease coverage by 16.95%.
The diff coverage is 92.42%.

@@            Coverage Diff             @@
##           master    #816       +/-   ##
==========================================
- Coverage   90.46%   73.5%   -16.96%     
==========================================
  Files          65      66        +1     
  Lines        6280    6345       +65     
==========================================
- Hits         5681    4664     -1017     
- Misses        599    1681     +1082

Impacted Files	Coverage Δ
src/gluonnlp/data/glue.py	22.72% <ø> (-75.92%)	⬇️
src/gluonnlp/data/intent_slot.py	92.42% <92.42%> (ø)
src/gluonnlp/model/train/cache.py	26.19% <0%> (-71.43%)	⬇️
src/gluonnlp/data/conll.py	36.84% <0%> (-63.16%)	⬇️
src/gluonnlp/embedding/evaluation.py	40.16% <0%> (-55.74%)	⬇️
src/gluonnlp/model/train/language_model.py	42.04% <0%> (-55.12%)	⬇️
src/gluonnlp/data/batchify/language_model.py	44.03% <0%> (-52.3%)	⬇️
src/gluonnlp/model/translation.py	20.63% <0%> (-50.8%)	⬇️
src/gluonnlp/data/sentiment.py	47.86% <0%> (-50.43%)	⬇️
... and 21 more

[mli]

Job PR-816/5 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-816/5/index.html