[tensorflow] Remove String tensor limitation for model output

[frankfliu]

Fixes #2053

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

Base: 72.08% // Head: 69.73% // Decreases project coverage by -2.35% ⚠️

Coverage data is based on head (9adb85f) compared to base (bb5073f).
Patch coverage: 68.95% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2056      +/-   ##
============================================
- Coverage     72.08%   69.73%   -2.36%     
- Complexity     5126     5990     +864     
============================================
  Files           473      600     +127     
  Lines         21970    26570    +4600     
  Branches       2351     2888     +537     
============================================
+ Hits          15838    18529    +2691     
- Misses         4925     6655    +1730     
- Partials       1207     1386     +179     

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)
...rc/main/java/ai/djl/modality/cv/output/Joints.java	71.42% <ø> (ø)
.../main/java/ai/djl/modality/cv/output/Landmark.java	100.00% <ø> (ø)
...main/java/ai/djl/modality/cv/output/Rectangle.java	72.41% <0.00%> (ø)
...i/djl/modality/cv/translator/BigGANTranslator.java	21.42% <0.00%> (-5.24%)	⬇️
...odality/cv/translator/BigGANTranslatorFactory.java	33.33% <0.00%> (+8.33%)	⬆️
.../cv/translator/InstanceSegmentationTranslator.java	0.00% <0.00%> (-86.59%)	⬇️
...nslator/InstanceSegmentationTranslatorFactory.java	7.14% <0.00%> (-11.04%)	⬇️
.../cv/translator/SemanticSegmentationTranslator.java	0.00% <0.00%> (ø)
... and 514 more

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