[Enhancement] Fix the usage of Pad in BERT

[eric-haibin-lin]

Description

Fix Pad constructor to avoid printing warnings

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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request head (pad@974918e). Click here to learn what that means.
The diff coverage is n/a.

[codecov]

Codecov Report

Merging #850 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #850   +/-   ##
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  Coverage   90.18%   90.18%           
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  Files          66       66           
  Lines        6345     6345           
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  Hits         5722     5722           
  Misses        623      623

[mli]

Found link check problems in job PR-850/2:
(line 19) broken https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/ - 404 Client Error: Not Found for url: https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/
(line 5) broken https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/ - 404 Client Error: Not Found for url: https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/
(line 5) broken https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/ - 404 Client Error: Not Found for url: https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/
(line 21) broken https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/ - 404 Client Error: Not Found for url: https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/
(line 442) broken https://www.aclweb.org/anthology/P02-1040.pdf)[1 - 404 Client Error: Not Found for url: https://www.aclweb.org/anthology/P02-1040.pdf)%5B1
(line 92) broken https://nlp.stanford.edu/pubs/glove.pdf)[2 - 404 Client Error: NOT FOUND for url: https://nlp.stanford.edu/pubs/glove.pdf)%5B2
(line 208) broken https://www.bioinf.jku.at/publications/older/2604.pdf)[3 - 404 Client Error: Not Found for url: https://www.bioinf.jku.at/publications/older/2604.pdf)%5B3

[mli]

Job PR-850/2 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-850/2/index.html