Fixes #1024, Add back string tensor support

[frankfliu]

Change-Id: I7b3f326669dec6739d24131d7507984390817226

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

Merging #1040 (ec45ecf) into master (6d951c9) will decrease coverage by 0.08%.
The diff coverage is 27.50%.

@@             Coverage Diff              @@
##             master    #1040      +/-   ##
============================================
- Coverage     70.09%   70.00%   -0.09%     
+ Complexity     5120     5119       -1     
============================================
  Files           504      504              
  Lines         22629    22654      +25     
  Branches       2375     2376       +1     
============================================
- Hits          15861    15859       -2     
- Misses         5498     5523      +25     
- Partials       1270     1272       +2     

Impacted Files	Coverage Δ
.../main/java/ai/djl/tensorflow/engine/TfNDArray.java	85.23% <0.00%> (ø)
...ain/java/ai/djl/tensorflow/engine/TfNDManager.java	88.28% <0.00%> (-1.41%)	⬇️
...ai/djl/tensorflow/engine/javacpp/JavacppUtils.java	67.74% <29.72%> (-9.56%)	⬇️
api/src/main/java/ai/djl/repository/Artifact.java	91.37% <0.00%> (-1.73%)	⬇️

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