make SpProcessor public

[lanking520]

Description

Per customer demand, make SpProcessor public to achieve better performance

[frankfliu]

We should make SpTokenizer.getProcessor() public as well.

I think creation of SpProcessor should always through SpTokenizer.
We can keep newInstance() and loadModel package protected.

Or, we merge newInstance() and SpTokenizer.loadModel(Path path, String prifix) into single API, and we can make it public

[codecov-commenter]

Codecov Report

Merging #1060 (6247148) into master (05ad89a) will decrease coverage by 0.36%.
The diff coverage is 63.31%.

@@             Coverage Diff              @@
##             master    #1060      +/-   ##
============================================
- Coverage     70.36%   70.00%   -0.37%     
- Complexity     5085     5213     +128     
============================================
  Files           501      510       +9     
  Lines         22415    23255     +840     
  Branches       2325     2489     +164     
============================================
+ Hits          15772    16279     +507     
- Misses         5403     5647     +244     
- Partials       1240     1329      +89     

Impacted Files	Coverage Δ
api/src/main/java/ai/djl/Device.java	71.11% <ø> (+0.89%)	⬆️
api/src/main/java/ai/djl/ndarray/NDArray.java	73.25% <0.00%> (-1.31%)	⬇️
.../java/ai/djl/repository/RepositoryFactoryImpl.java	76.74% <0.00%> (-1.83%)	⬇️
.../src/main/java/ai/djl/repository/zoo/ModelZoo.java	54.00% <0.00%> (-18.81%)	⬇️
api/src/main/java/ai/djl/util/Platform.java	45.65% <0.00%> (-3.19%)	⬇️
.../main/java/ai/djl/zero/cv/ImageClassification.java	0.00% <0.00%> (ø)
...rc/main/java/ai/djl/sentencepiece/SpProcessor.java	77.27% <ø> (ø)
...rc/main/java/ai/djl/sentencepiece/SpTokenizer.java	63.63% <ø> (ø)
...ine/src/main/java/ai/djl/mxnet/engine/MxModel.java	61.11% <ø> (-1.06%)	⬇️
...src/main/java/ai/djl/mxnet/engine/MxNDArrayEx.java	87.77% <ø> (ø)
... and 128 more

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