[tensorflow] Fixes string tensor implemenation

[frankfliu]

1. Add create(String[], Shape) to NDArray interface
2. Implemented full string tensor for TF
3. Fixed Onnx NDManager.create(String) implementation

Change-Id: If51e55373ac800bd2df0ed1deb937fc60fca8b70

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 #1251 (67358e3) into master (4f633b3) will increase coverage by 0.20%.
The diff coverage is 92.00%.

@@             Coverage Diff              @@
##             master    #1251      +/-   ##
============================================
+ Coverage     70.37%   70.57%   +0.20%     
- Complexity     5069     5090      +21     
============================================
  Files           485      485              
  Lines         22248    22286      +38     
  Branches       2379     2384       +5     
============================================
+ Hits          15656    15729      +73     
+ Misses         5386     5349      -37     
- Partials       1206     1208       +2     

Impacted Files	Coverage Δ
...pi/src/main/java/ai/djl/ndarray/BaseNDManager.java	67.72% <ø> (+0.42%)	⬆️
...i/src/main/java/ai/djl/ndarray/NDArrayAdapter.java	16.14% <ø> (ø)
...n/java/ai/djl/onnxruntime/engine/OrtNDManager.java	72.22% <0.00%> (-4.25%)	⬇️
...src/main/java/ai/djl/pytorch/engine/PtNDArray.java	81.86% <ø> (ø)
...ain/java/ai/djl/onnxruntime/engine/OrtNDArray.java	70.37% <66.66%> (-2.36%)	⬇️
api/src/main/java/ai/djl/ndarray/NDArray.java	77.27% <100.00%> (+1.27%)	⬆️
api/src/main/java/ai/djl/ndarray/NDManager.java	58.55% <100.00%> (+1.12%)	⬆️
...e/src/main/java/ai/djl/mxnet/engine/MxNDArray.java	86.82% <100.00%> (ø)
.../main/java/ai/djl/tensorflow/engine/TfNDArray.java	85.76% <100.00%> (+1.47%)	⬆️
...ain/java/ai/djl/tensorflow/engine/TfNDManager.java	89.31% <100.00%> (+2.42%)	⬆️
... and 9 more

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