[api] Added Early stopping configuration (#38)

[jagodevreede]

Description

This PR implements a listener for early stopping as described in #38. And takes the code @gforman44 and implements that in a TrainingListener

resolves #38

[zachgk]

Thank you for contributing this @jagodevreede! I'm sure many training users will appreciate it. Your PR looks pretty good, but I left a few comments.

I'll also run the CI. It will probably complain from our code formatter and checkstyle. See http://docs.djl.ai/docs/development/development_guideline.html#coding-conventions for some explanations and how to check it locally as well

[jagodevreede]

Local build is happy now, let's see if CI agrees with that. Also fixed your comments.

[jagodevreede]

I was able to reproduce the failing ci build locally with JDK11. Fixed it in commit 402df41

[codecov-commenter]

Codecov Report

Attention: 1361 lines in your changes are missing coverage. Please review.

Comparison is base (bb5073f) 72.08% compared to head (402df41) 72.04%.
Report is 898 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2806      +/-   ##
============================================
- Coverage     72.08%   72.04%   -0.05%     
- Complexity     5126     7106    +1980     
============================================
  Files           473      703     +230     
  Lines         21970    31751    +9781     
  Branches       2351     3294     +943     
============================================
+ Hits          15838    22875    +7037     
- Misses         4925     7303    +2378     
- Partials       1207     1573     +366     

Files	Coverage Δ
...ava/ai/djl/inference/streaming/StreamingBlock.java	100.00% <100.00%> (ø)
api/src/main/java/ai/djl/metric/Dimension.java	100.00% <100.00%> (ø)
api/src/main/java/ai/djl/metric/Unit.java	100.00% <100.00%> (ø)
api/src/main/java/ai/djl/modality/audio/Audio.java	100.00% <100.00%> (ø)
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)
...ava/ai/djl/modality/cv/output/DetectedObjects.java	96.29% <100.00%> (+1.29%)	⬆️
...rc/main/java/ai/djl/modality/cv/output/Joints.java	71.42% <100.00%> (ø)
.../main/java/ai/djl/modality/cv/output/Landmark.java	100.00% <ø> (ø)
...i/djl/modality/cv/transform/RandomResizedCrop.java	94.11% <100.00%> (+5.22%)	⬆️
... and 226 more

... and 373 files with indirect coverage changes

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