allow empty batch data

[patins1]

Description

This fix handles the edge case of flattening a batch of zero size. It is a good quality metric for a software function to be able to handle zero data input even if the calling function could detect this case and create a fake expected output (that was my work-around before).

Before this fix, this exception would have been thrown:

Caused by: ai.djl.engine.EngineException: cannot reshape tensor of 0 elements into shape [0, -1] because the unspecified dimension size -1 can be any value and is ambiguous
at ai.djl.pytorch.jni.PyTorchLibrary.torchReshape(Native Method)

[zachgk]

The change looks good @patins1, but it seems like you are failing the formatter check. Can you run ./gradlew formatJava (see docs)?

[codecov-commenter]

Codecov Report

Merging #1569 (a652145) into master (bb5073f) will decrease coverage by 1.25%.
The diff coverage is 52.95%.

@@             Coverage Diff              @@
##             master    #1569      +/-   ##
============================================
- Coverage     72.08%   70.83%   -1.26%     
- Complexity     5126     5394     +268     
============================================
  Files           473      503      +30     
  Lines         21970    23579    +1609     
  Branches       2351     2561     +210     
============================================
+ Hits          15838    16703     +865     
- Misses         4925     5597     +672     
- Partials       1207     1279      +72     

Impacted Files	Coverage Δ
api/src/main/java/ai/djl/modality/cv/Image.java	69.23% <ø> (-4.11%)	⬇️
...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%)	⬇️
...pi/src/main/java/ai/djl/ndarray/BytesSupplier.java	54.54% <0.00%> (-12.13%)	⬇️
...ain/java/ai/djl/ndarray/index/dim/NDIndexPick.java	100.00% <ø> (ø)
api/src/main/java/ai/djl/nn/Blocks.java	75.00% <0.00%> (-25.00%)	⬇️
... and 230 more

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