[djl-bench] Add warmup to benchmark

[frankfliu]

Change-Id: I3f5e5ec3bc6d9c4f2c4ad09c6695775f50981ebd

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 #1152 (bca728c) into master (0df8265) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1152      +/-   ##
============================================
- Coverage     70.26%   70.25%   -0.01%     
+ Complexity     5339     5337       -2     
============================================
  Files           513      513              
  Lines         23664    23672       +8     
  Branches       2552     2552              
============================================
+ Hits          16627    16631       +4     
- Misses         5674     5676       +2     
- Partials       1363     1365       +2     

Impacted Files	Coverage Δ
.../main/java/ai/djl/benchmark/AbstractBenchmark.java	91.27% <100.00%> (-0.06%)	⬇️
...mark/src/main/java/ai/djl/benchmark/Benchmark.java	63.41% <100.00%> (+2.88%)	⬆️
.../java/ai/djl/benchmark/MultithreadedBenchmark.java	77.01% <100.00%> (+1.70%)	⬆️
api/src/main/java/ai/djl/repository/Artifact.java	87.80% <0.00%> (-1.63%)	⬇️
...c/main/java/ai/djl/mxnet/engine/MxSymbolBlock.java	82.01% <0.00%> (-1.44%)	⬇️

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