[SentencePiece] Make SpProcessor public

[frankfliu]

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

Patch coverage: 54.49% and project coverage change: +0.06% 🎉

Comparison is base (bb5073f) 72.08% compared to head (fbf465c) 72.15%.
Report is 877 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2765      +/-   ##
============================================
+ Coverage     72.08%   72.15%   +0.06%     
- Complexity     5126     7056    +1930     
============================================
  Files           473      700     +227     
  Lines         21970    31434    +9464     
  Branches       2351     3249     +898     
============================================
+ Hits          15838    22681    +6843     
- Misses         4925     7205    +2280     
- Partials       1207     1548     +341     

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 370 files with indirect coverage changes

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