[serving] auto detect number of GPUs

[frankfliu]

Change-Id: Ief11f4df5d7f1622d42385842dacadefc118e7d4

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 #1132 (924a408) into master (21bf5fe) will decrease coverage by 0.04%.
The diff coverage is 57.69%.

@@             Coverage Diff              @@
##             master    #1132      +/-   ##
============================================
- Coverage     70.19%   70.15%   -0.05%     
- Complexity     5323     5327       +4     
============================================
  Files           513      513              
  Lines         23622    23632      +10     
  Branches       2529     2536       +7     
============================================
- Hits          16582    16578       -4     
- Misses         5695     5708      +13     
- Partials       1345     1346       +1     

Impacted Files	Coverage Δ
...src/main/java/ai/djl/repository/JarRepository.java	88.63% <ø> (-0.50%)	⬇️
.../main/java/ai/djl/repository/SimpleRepository.java	80.76% <ø> (-0.25%)	⬇️
...in/java/ai/djl/repository/SimpleUrlRepository.java	71.66% <ø> (-0.92%)	⬇️
api/src/main/java/ai/djl/repository/Artifact.java	86.66% <50.00%> (-4.72%)	⬇️
...ving/src/main/java/ai/djl/serving/ModelServer.java	50.00% <57.14%> (-1.47%)	⬇️
api/src/main/java/ai/djl/repository/Metadata.java	86.36% <100.00%> (ø)
...gine/src/main/java/ai/djl/dlr/engine/DlrModel.java	53.12% <0.00%> (-3.13%)	⬇️
api/src/main/java/ai/djl/util/Platform.java	42.30% <0.00%> (-1.93%)	⬇️
...g/src/main/java/ai/djl/serving/util/Connector.java	55.40% <0.00%> (-1.36%)	⬇️
... and 6 more

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