[pytorch] Implement padding to bert tokenizer

[frankfliu]

Change-Id: I033baaaa1753570e62e11d2096c6ef03cd1aa48f

Description

Brief description of what this PR is about

• If this change is a backward incompatible change, why must this change be made?
• Interesting edge cases to note here

[codecov-commenter]

Codecov Report

Merging #1328 (354ce8d) into master (bb5073f) will decrease coverage by 0.48%.
The diff coverage is 55.44%.

@@             Coverage Diff              @@
##             master    #1328      +/-   ##
============================================
- Coverage     72.08%   71.60%   -0.49%     
- Complexity     5126     5142      +16     
============================================
  Files           473      478       +5     
  Lines         21970    22256     +286     
  Branches       2351     2385      +34     
============================================
+ Hits          15838    15937      +99     
- Misses         4925     5108     +183     
- Partials       1207     1211       +4     

Impacted Files	Coverage Δ
...i/djl/modality/cv/translator/BigGANTranslator.java	21.42% <ø> (-5.24%)	⬇️
...odality/cv/translator/BigGANTranslatorFactory.java	33.33% <0.00%> (+8.33%)	⬆️
...nslator/InstanceSegmentationTranslatorFactory.java	14.28% <0.00%> (-3.90%)	⬇️
.../modality/cv/translator/YoloTranslatorFactory.java	8.33% <0.00%> (-1.67%)	⬇️
...i/djl/modality/cv/translator/YoloV5Translator.java	5.69% <0.00%> (ø)
...odality/cv/translator/YoloV5TranslatorFactory.java	8.33% <0.00%> (-1.67%)	⬇️
...n/java/ai/djl/modality/nlp/bert/BertTokenizer.java	92.68% <0.00%> (-2.32%)	⬇️
...modality/nlp/embedding/TrainableWordEmbedding.java	45.00% <0.00%> (-3.34%)	⬇️
...pi/src/main/java/ai/djl/ndarray/BytesSupplier.java	54.54% <0.00%> (-12.13%)	⬇️
...i/src/main/java/ai/djl/ndarray/NDArrayAdapter.java	15.23% <0.00%> (-0.83%)	⬇️
... and 83 more

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[zachgk]

This PR is fine, but we need to rename maxLength because that implies it is for truncating instead of padding

[frankfliu]

This PR is fine, but we need to rename maxLength because that implies it is for truncating instead of padding

maxLength should be fine, it imply both padding and truncation