[Bug][Fix][WIP] Fix pre-layernormalization in Transformer

[sxjscience]

Description

Fix the additional of the residual connection. The previous implementation was not correct. I'm rerunning the Transformer-Big-pre-ln experiment.

@yongyi-wu

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]

Codecov Report

Merging #1488 (3c7c4c1) into master (c582b64) will increase coverage by 3.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1488      +/-   ##
==========================================
+ Coverage   81.98%   85.19%   +3.21%     
==========================================
  Files          52       52              
  Lines        6909     6822      -87     
==========================================
+ Hits         5664     5812     +148     
+ Misses       1245     1010     -235     

Impacted Files	Coverage Δ
src/gluonnlp/data/batchify.py	88.72% <ø> (ø)
src/gluonnlp/layers.py	87.15% <100.00%> (+0.03%)	⬆️
src/gluonnlp/models/transformer.py	98.93% <100.00%> (-0.01%)	⬇️
conftest.py	76.31% <0.00%> (-8.69%)	⬇️
src/gluonnlp/data/loading.py	75.75% <0.00%> (-7.64%)	⬇️
src/gluonnlp/utils/lazy_imports.py	58.42% <0.00%> (-2.25%)	⬇️
src/gluonnlp/utils/misc.py	52.51% <0.00%> (-1.06%)	⬇️
src/gluonnlp/data/tokenizers/yttm.py	81.73% <0.00%> (-1.02%)	⬇️
src/gluonnlp/data/tokenizers/spacy.py	65.33% <0.00%> (-0.91%)	⬇️
src/gluonnlp/data/tokenizers/huggingface.py	71.06% <0.00%> (-0.78%)	⬇️
... and 22 more

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[yongyi-wu]

Looks good— it seems all issues related to pre-norm and skip connection have been fixed.

[github-actions]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1488/fix_pre_ln/index.html

[sxjscience]

I noticed that the performance becomes worse after I changed the implementation. Still investigating the issue.