[api] Support encode/decode String tensor

[frankfliu]

Fixes: #3033

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

Attention: Patch coverage is 90.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 72.31%. Comparing base (b6b8729) to head (7f786dd).
Report is 1 commits behind head on master.

Files	Patch %	Lines
...src/main/java/ai/djl/pytorch/engine/PtNDArray.java	60.00%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3034      +/-   ##
============================================
+ Coverage     72.29%   72.31%   +0.01%     
- Complexity     7293     7298       +5     
============================================
  Files           722      722              
  Lines         32512    32533      +21     
  Branches       3396     3401       +5     
============================================
+ Hits          23506    23525      +19     
+ Misses         7386     7385       -1     
- Partials       1620     1623       +3     

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