Predict_proba functionality to Random Forest Classifier

[skywardfire1]

Checklist

• [yes] My branch is up-to-date with development branch.
• [yes] Everything works and tested on latest stable Rust.
• [yes] Coverage and Linting have been applied

This PR adds:

1. Proper probability estimation to Decision Tree Classifier, matching scikit-learn's predict_proba behavior.
2. Predict_proba functionality to Random Forest Classifier, also in scikit-learn style, which was the goal.

Previously, probability predictions from Decision Tree returned one-hot encoded vectors (1.0 for the majority class, 0.0 for others), which did not reflect actual class distributions in leaf nodes.
While we could still use it in Random Forest, this approach would not provide calibrated probability estimates.

Changes:

1. Node structure extended: Added class_distribution: Vec<usize> field to store the class histogram in each node. This data was already being computed during training but was not persisted.
2. DecisionTreeClassifier: Added predict_proba_for_row_real() method that returns proper probability distributions based on leaf class counts. The original predict_proba() method remains unchanged for backward compatibility. Hope, we will obsolete it one day.
3. RandomForestClassifier: Added public predict_proba() method that averages probability distributions from all trees (scikit-learn style), rather than averaging hard class predictions.
4. Testing: Added 5 new tests covering:
   • Probability distributions summing to 1.0
   • Correct class ordering in predictions
   • Mixed-class leaf handling
   • Forest-level probability averaging
5. Debug assertions: Added 3 debug_assert_eq!, just to feel better.

Backward Compatibility:
No breaking changes. All existing APIs remain intact.

Note:
Current predict_proba function returns Vec<Vec<f64>>, not DenseMatrix<f64>, since I didn't find any examples on what is the default behavior or standard for this.

[codecov]

Codecov Report

❌ Patch coverage is 56.36364% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.69%. Comparing base (70d8a0f) to head (fc969fe).
⚠️ Report is 10 commits behind head on development.

Files with missing lines	Patch %	Lines
src/tree/decision_tree_classifier.rs	62.16%	14 Missing ⚠️
src/ensemble/random_forest_classifier.rs	44.44%	10 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #360      +/-   ##
===============================================
- Coverage        45.59%   44.69%   -0.90%     
===============================================
  Files               93       95       +2     
  Lines             8034     8054      +20     
===============================================
- Hits              3663     3600      -63     
- Misses            4371     4454      +83     

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[skywardfire1]

applied auto formatting.

[Mec-iS]

wow. thank you. will take a look asap

[skywardfire1]

some additional info. This is how it looks like in my project

labels - True:      [1, 1, 4, 0, 0, 5, 3, 0, 0, 5, 0, 0, 0, 0, 0, 4, 4, 1, 2, 3, 5, 1, 4, 0, 3, 1, 0, 0, 3, 3, 0, 0, 0, 4, 5, 1, 1, 0, 0, 1, 5, 2, 4, 4, 0, 0, 1, 1, 3, 4, 0, 0, 4, 2, 2, 3, 4, 5, 5, 0, 0, 5, 0, 0, 0, 4, 4, 1, 5]
labels - Predicted: [5, 1, 4, 0, 0, 5, 2, 0, 0, 5, 0, 0, 4, 0, 0, 4, 4, 5, 2, 3, 5, 5, 4, 0, 3, 3, 0, 0, 3, 3, 0, 0, 0, 4, 5, 1, 1, 0, 0, 1, 5, 2, 4, 1, 0, 0, 1, 5, 3, 4, 0, 0, 4, 4, 4, 3, 4, 5, 5, 0, 0, 5, 0, 0, 0, 4, 4, 1, 1]

Primary probabilities (first 5 samples):
    [[0.0, 0.2, 0.0, 0.0, 0.0, 0.8, 0.0, 0.0],
     [0.0, 0.9, 0.0, 0.0, 0.0, 0.1, 0.0, 0.0],
     [0.1, 0.0, 0.0, 0.0, 0.9, 0.0, 0.0, 0.0],
     [0.9, 0.0, 0.0, 0.0, 0.1, 0.0, 0.0, 0.0],
     [1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]] 

And [1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0] result is funny to me since the answer is correct, but so shy

[Mec-iS]

this looks OK.

it would be nice to have a test using the iris dataset like this one:

    #[cfg_attr(
        all(target_arch = "wasm32", not(target_os = "wasi")),
        wasm_bindgen_test::wasm_bindgen_test
    )]
    #[test]
    fn fit_predict_iris_oob() { 
        ...

so the results can be checked on known results.

[Mec-iS]

maybe a little better error handling like:

  pub fn predict_proba(&self, x: &X) -> Result<Vec<Vec<f64>>, Failed> {                                                 
      let (n, _) = x.shape();                                                                                           
      let mut result = Vec::with_capacity(n);                                                                           
      for i in 0..n {                                                                                                   
          result.push(self.predict_proba_for_row(x, i));                                                                
      }                                                                                                                 
      Ok(result)                                                                                                        
  }  

Rest looks good.

[Mec-iS]

check out my comments

[skywardfire1]

I revisited the tests. As said in commit comment, now there are 2 tests. The first one uses Iris dataset from the beginning of the file, and performs 4 checks.

Everything builds and works perfectly at my side, clippy and fmt --all shows no issues, so I have no idea why builds fail.

What about error handling. I can physically add error checks but it seems useless since there are no operations could possibly return an error if the user doesn't break the API.

[skywardfire1]

still, checks don't look good. I offer to throw this PR away, I'll make another one soon after, with same functionality.

[Mec-iS]

no problem. proceed as you find right