[MAINTENANCE] Move ParallelBigRNN into model api

[liuzh47]

Description

There is a class ParallelBigRNN defined in the large_word_language_model.py script. I think it should be merged to the model api like class ParallelTransformer. Additionally, I need to reuse this class in some estimator 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

[codecov]

Codecov Report

Merging #1118 into master will decrease coverage by 0.16%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master    #1118      +/-   ##
==========================================
- Coverage   88.34%   88.18%   -0.17%     
==========================================
  Files          66       66              
  Lines        6290     6306      +16     
==========================================
+ Hits         5557     5561       +4     
- Misses        733      745      +12

Impacted Files	Coverage Δ
src/gluonnlp/model/train/language_model.py	88.51% <25%> (-5.27%)	⬇️

[mli]

Job PR-1118/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1118/1/index.html

[mli]

Job PR-1118/2 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1118/2/index.html

[mli]

Job PR-1118/3 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1118/3/index.html

[mli]

Job PR-1118/4 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1118/4/index.html

[mli]

Job PR-1118/5 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1118/5/index.html

[leezu]

LGTM, but would be good to introduce a test case that runs large_word_language_model.py in this PR or follow-up Estimator PR