LSTM : padding when the size of input is 2

[takanori-ugai]

This is a bug fix patch.

LSTM requires two inputs.
Tested with https://d2l.djl.ai/chapter_recurrent-modern/lstm.html and https://d2l.djl.ai/chapter_recurrent-modern/deep-rnn.html

[codecov-commenter]

Codecov Report

Merging #2000 (4696bc2) into master (bb5073f) will decrease coverage by 2.21%.
The diff coverage is 68.60%.

@@             Coverage Diff              @@
##             master    #2000      +/-   ##
============================================
- Coverage     72.08%   69.87%   -2.22%     
- Complexity     5126     5915     +789     
============================================
  Files           473      586     +113     
  Lines         21970    26222    +4252     
  Branches       2351     2848     +497     
============================================
+ Hits          15838    18323    +2485     
- Misses         4925     6517    +1592     
- Partials       1207     1382     +175     

Impacted Files	Coverage Δ
api/src/main/java/ai/djl/modality/cv/Image.java	69.23% <ø> (-4.11%)	⬇️
...rc/main/java/ai/djl/modality/cv/MultiBoxPrior.java	76.00% <ø> (ø)
...rc/main/java/ai/djl/modality/cv/output/Joints.java	71.42% <ø> (ø)
.../main/java/ai/djl/modality/cv/output/Landmark.java	100.00% <ø> (ø)
...main/java/ai/djl/modality/cv/output/Rectangle.java	72.41% <0.00%> (ø)
...i/djl/modality/cv/translator/BigGANTranslator.java	21.42% <0.00%> (-5.24%)	⬇️
...odality/cv/translator/BigGANTranslatorFactory.java	33.33% <0.00%> (+8.33%)	⬆️
.../cv/translator/InstanceSegmentationTranslator.java	0.00% <0.00%> (-86.59%)	⬇️
...nslator/InstanceSegmentationTranslatorFactory.java	7.14% <0.00%> (-11.04%)	⬇️
.../cv/translator/SemanticSegmentationTranslator.java	0.00% <0.00%> (ø)
... and 500 more

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