[spark] Change implicit conversions to explicit

[xyang16]

Description

Brief description of what this PR is about

Change implicit conversions to asScala() to avoid confusion and hard-to-debug issues.

[codecov-commenter]

Codecov Report

Patch coverage: 71.75% and project coverage change: +1.53 🎉

Comparison is base (bb5073f) 72.08% compared to head (dfbc7eb) 73.62%.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2595      +/-   ##
============================================
+ Coverage     72.08%   73.62%   +1.53%     
- Complexity     5126     6960    +1834     
============================================
  Files           473      684     +211     
  Lines         21970    30396    +8426     
  Branches       2351     3149     +798     
============================================
+ Hits          15838    22379    +6541     
- Misses         4925     6486    +1561     
- Partials       1207     1531     +324     

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)
.../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 228 more

... and 353 files with indirect coverage changes

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