Match OpenCV to grayscale algorithm, #964

[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]

Codecov Report

Merging #965 (48d9fbf) into master (c17c15a) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #965      +/-   ##
============================================
- Coverage     70.24%   70.20%   -0.04%     
+ Complexity     5091     5088       -3     
============================================
  Files           502      502              
  Lines         22479    22479              
  Branches       2348     2348              
============================================
- Hits          15790    15782       -8     
- Misses         5444     5451       +7     
- Partials       1245     1246       +1     

Impacted Files	Coverage Δ	Complexity Δ
.../java/ai/djl/modality/cv/BufferedImageFactory.java	72.44% <100.00%> (ø)	10.00 <0.00> (ø)
api/src/main/java/ai/djl/training/Trainer.java	91.83% <0.00%> (-3.07%)	33.00% <0.00%> (-1.00%)
api/src/main/java/ai/djl/inference/Predictor.java	87.50% <0.00%> (-2.89%)	23.00% <0.00%> (-1.00%)
.../src/main/java/ai/djl/repository/zoo/Criteria.java	89.84% <0.00%> (-1.57%)	24.00% <0.00%> (-1.00%)

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