[fix] Set past_kv name for corner case.

[KexinFeng]

Description

1. Add documentation and clarify some naming
2. Set tuple name for pastKeyValues, when the init inference call to gpt2 has not-null kvCache input. Previously this case was missed and would cause error.

[codecov-commenter]

Codecov Report

Patch coverage: 54.72% and project coverage change: +0.01% 🎉

Comparison is base (bb5073f) 72.08% compared to head (e85efd0) 72.10%.
Report is 858 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2722      +/-   ##
============================================
+ Coverage     72.08%   72.10%   +0.01%     
- Complexity     5126     7028    +1902     
============================================
  Files           473      698     +225     
  Lines         21970    31270    +9300     
  Branches       2351     3225     +874     
============================================
+ Hits          15838    22548    +6710     
- Misses         4925     7189    +2264     
- Partials       1207     1533     +326     

Files Changed	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)
.../main/java/ai/djl/modality/cv/output/Landmark.java	100.00% <ø> (ø)
...djl/modality/cv/transform/RandomFlipLeftRight.java	25.00% <0.00%> (-25.00%)	⬇️
...djl/modality/cv/transform/RandomFlipTopBottom.java	25.00% <0.00%> (-25.00%)	⬇️
...i/djl/modality/cv/translator/BigGANTranslator.java	21.42% <0.00%> (-5.24%)	⬇️
.../modality/cv/translator/ImageFeatureExtractor.java	0.00% <0.00%> (ø)
.../ai/djl/modality/cv/translator/YoloTranslator.java	27.77% <0.00%> (+18.95%)	⬆️
...ain/java/ai/djl/modality/cv/util/NDImageUtils.java	67.10% <0.00%> (+7.89%)	⬆️
api/src/main/java/ai/djl/modality/nlp/Decoder.java	63.63% <ø> (ø)
... and 226 more

... and 368 files with indirect coverage changes

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