Make BufferedImageFactory.fromNDArray synchronous

[zachgk]

I made this PR based on this comment by @steinhae in #1278.

I also ran a quick experiment to verify the findings:

try (NDManager manager = NDManager.newBaseManager()) {
  NDArray array = manager.randomInteger(0, 255, new Shape(3, 400, 400), DataType.INT32).toType(DataType.UINT8, true);
  ImageFactory factory = ImageFactory.getInstance();
  for (int i=0; i<5000; i++) {
      factory.fromNDArray(array);
  }
}

Found that this change would reduce the time to run this test from about 62 seconds to about 22 seconds.

[codecov-commenter]

Codecov Report

Merging #1339 (03cdad5) into master (bb5073f) will decrease coverage by 0.53%.
The diff coverage is 54.92%.

@@             Coverage Diff              @@
##             master    #1339      +/-   ##
============================================
- Coverage     72.08%   71.55%   -0.54%     
- Complexity     5126     5145      +19     
============================================
  Files           473      478       +5     
  Lines         21970    22295     +325     
  Branches       2351     2394      +43     
============================================
+ Hits          15838    15954     +116     
- Misses         4925     5122     +197     
- Partials       1207     1219      +12     

Impacted Files	Coverage Δ
.../java/ai/djl/modality/cv/BufferedImageFactory.java	72.68% <ø> (+0.23%)	⬆️
...i/djl/modality/cv/translator/BigGANTranslator.java	21.42% <ø> (-5.24%)	⬇️
...odality/cv/translator/BigGANTranslatorFactory.java	33.33% <0.00%> (+8.33%)	⬆️
...nslator/InstanceSegmentationTranslatorFactory.java	14.28% <0.00%> (-3.90%)	⬇️
.../modality/cv/translator/YoloTranslatorFactory.java	8.33% <0.00%> (-1.67%)	⬇️
...i/djl/modality/cv/translator/YoloV5Translator.java	5.69% <0.00%> (ø)
...odality/cv/translator/YoloV5TranslatorFactory.java	8.33% <0.00%> (-1.67%)	⬇️
...n/java/ai/djl/modality/nlp/bert/BertTokenizer.java	92.68% <0.00%> (-2.32%)	⬇️
...modality/nlp/embedding/TrainableWordEmbedding.java	45.00% <0.00%> (-3.34%)	⬇️
...pi/src/main/java/ai/djl/ndarray/BytesSupplier.java	54.54% <0.00%> (-12.13%)	⬇️
... and 92 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fa7ff1d...03cdad5. Read the comment docs.