Make benchmark a standalone module

[frankfliu]

Change-Id: I3b169b2c27239ae57be09ca63cb24e032d32e4f6

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 #1043 (95df765) into master (50e4c43) will increase coverage by 0.33%.
The diff coverage is 73.97%.

@@             Coverage Diff              @@
##             master    #1043      +/-   ##
============================================
+ Coverage     70.02%   70.36%   +0.33%     
- Complexity     5118     5168      +50     
============================================
  Files           504      508       +4     
  Lines         22634    23084     +450     
  Branches       2378     2429      +51     
============================================
+ Hits          15850    16242     +392     
- Misses         5514     5536      +22     
- Partials       1270     1306      +36     

Impacted Files	Coverage Δ
...mark/src/main/java/ai/djl/benchmark/Benchmark.java	62.50% <50.00%> (ø)
...mark/src/main/java/ai/djl/benchmark/Arguments.java	78.08% <70.96%> (ø)
.../main/java/ai/djl/benchmark/AbstractBenchmark.java	91.12% <95.65%> (ø)
.../java/ai/djl/benchmark/MultithreadedBenchmark.java	73.23% <100.00%> (ø)
.../main/java/ai/djl/serving/wlm/WorkLoadManager.java	64.76% <0.00%> (-5.72%)	⬇️
api/src/main/java/ai/djl/inference/Predictor.java	87.50% <0.00%> (-2.89%)	⬇️
... and 5 more

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