[Backbone] Add gluon_en_cased_bert_base_v1 pretrained via MLM + SOP

[ZiyueHuang]

Description

@sxjscience

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

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1411/bert_url/index.html

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1411/bert_url/index.html

[codecov[bot]]

Codecov Report

Merging #1411 into master will increase coverage by 0.18%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1411      +/-   ##
==========================================
+ Coverage   85.12%   85.31%   +0.18%     
==========================================
  Files          53       53              
  Lines        6946     6946              
==========================================
+ Hits         5913     5926      +13     
+ Misses       1033     1020      -13     

Impacted Files	Coverage Δ
src/gluonnlp/models/bert.py	94.80% <ø> (ø)
src/gluonnlp/attention_cell.py	80.31% <0.00%> (-0.40%)	:arrow_down:
src/gluonnlp/data/loading.py	83.39% <0.00%> (+5.28%)	:arrow_up:

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[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1411/bert_url/index.html