[api] Adds utility method to Model for accessing properties

[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]

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Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 72.29%. Comparing base (19e4302) to head (5226348).
Report is 449 commits behind head on master.

Files with missing lines	Patch %	Lines
api/src/main/java/ai/djl/Model.java	50.00%	2 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##             master    #3007   +/-   ##
=========================================
  Coverage     72.29%   72.29%           
- Complexity     7284     7287    +3     
=========================================
  Files           722      722           
  Lines         32483    32491    +8     
  Branches       3388     3390    +2     
=========================================
+ Hits          23484    23490    +6     
- Misses         7384     7385    +1     
- Partials       1615     1616    +1     

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