[pytorch] Adds image classification resnet18 base model to model zoo

[frankfliu]

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: 70.21% // Decreases project coverage by -1.86% ⚠️

Coverage data is based on head (2bed909) compared to base (bb5073f).
Patch coverage: 71.74% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2079      +/-   ##
============================================
- Coverage     72.08%   70.21%   -1.87%     
- Complexity     5126     6077     +951     
============================================
  Files           473      605     +132     
  Lines         21970    26924    +4954     
  Branches       2351     2917     +566     
============================================
+ Hits          15838    18906    +3068     
- Misses         4925     6604    +1679     
- Partials       1207     1414     +207     

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%)	⬇️
.../modality/cv/translator/ImageFeatureExtractor.java	0.00% <0.00%> (ø)
.../ai/djl/modality/cv/translator/YoloTranslator.java	27.77% <0.00%> (+18.95%)	⬆️
...modality/cv/translator/wrapper/FileTranslator.java	44.44% <ø> (ø)
...y/cv/translator/wrapper/InputStreamTranslator.java	44.44% <ø> (ø)
... and 526 more

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