Add harmonic mean combination

[martin-gaievski]

Description

Adding harmonic mean technique (it's a reciprocal of the arithmetic mean of the reciprocals, more details here ). It does support weighted mean similarly to arithmetic mean. Example of pipeline with processor config:

{
    "description": "Post processor for hybrid search",
    "phase_results_processors": [
        {
            "normalization-processor": {}, 
            "combination": {
                 "technique": "harmonic_mean",
                  "parameters": {
                        "weights": [
                            0.4, 0.7
                        ]
                    }
             }
        }
    ]
}

Issues Resolved

#228, part of solution for #126

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[codecov]

Codecov Report

Merging #238 (2527fee) into feature/normalization (6ad641a) will increase coverage by 3.59%.
The diff coverage is 93.93%.

❗ Current head 2527fee differs from pull request most recent head 8936d5d. Consider uploading reports for the commit 8936d5d to get more accurate results

@@                     Coverage Diff                     @@
##             feature/normalization     #238      +/-   ##
===========================================================
+ Coverage                    82.43%   86.02%   +3.59%     
- Complexity                     323      331       +8     
===========================================================
  Files                           26       27       +1     
  Lines                          979      959      -20     
  Branches                       153      150       -3     
===========================================================
+ Hits                           807      825      +18     
+ Misses                         108       69      -39     
- Partials                        64       65       +1     

Files Changed	Coverage Δ
...processor/combination/ScoreCombinationFactory.java	100.00% <ø> (ø)
...ination/HarmonicMeanScoreCombinationTechnique.java	94.11% <88.88%> (+94.11%)	⬆️
...combination/AbstractScoreCombinationTechnique.java	95.45% <95.45%> (ø)
...ation/ArithmeticMeanScoreCombinationTechnique.java	89.47% <100.00%> (-2.84%)	⬇️

[heemin32]

Can we rename local variable name to a different one to avoid future mistake?

[martin-gaievski]

ack

[navneet1v]

I am not seeing this class as an abstract class, for me its more like having some utility functions. All we are doing here is just doing some utility things. I would rather move this class as a utility class like ScoreCombinationUtility.

Also, the function: getSupportedParams, seems best for an interface and not for an abstract class.

[martin-gaievski]

Utility class was my second option, my concern was that those methods will be visible. We can put utility class into same package and at least not make methods private. Ack for the method with supported params, it should go to the interface especially if we'll get rid of the abstract class.

[heemin32]

For me, exposing those utility methods is not a big concern.

[martin-gaievski]

refactored abstract class into util class as suggested

[navneet1v]

We can make the utility class as package private. I think thats a good thing to do.

[martin-gaievski]

CI will fail for some time until compilation errors are fixed in knn, related PR opensearch-project/k-NN#1014