[Performance][WIP] Use boolean mask

[sxjscience]

Description

Switch to use boolean mask, which helps reduce the memory consumption of the models.

Checklist

Essentials

• [x] PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• [x] Changes are complete (i.e. I finished coding on this PR)
• [x] All changes have test coverage
• [x] Code is well-documented

cc @dmlc/gluon-nlp-team

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1403/boolean_mask/index.html

[codecov[bot]]

Codecov Report

Merging #1403 into master will increase coverage by 0.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1403      +/-   ##
==========================================
+ Coverage   82.17%   82.26%   +0.08%     
==========================================
  Files          53       53              
  Lines        6946     6946              
==========================================
+ Hits         5708     5714       +6     
+ Misses       1238     1232       -6     

Impacted Files	Coverage Δ
src/gluonnlp/attention_cell.py	79.92% <100.00%> (ø)
src/gluonnlp/data/loading.py	83.39% <0.00%> (+2.26%)	:arrow_up:

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[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1403/boolean_mask/index.html

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1403/boolean_mask/index.html

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1403/boolean_mask/index.html