REFACTOR: Break down the fit method in MapieQuantileRegressor into mu…

[jawadhussein462]

Description

The objective of this PR is to refactor the fit method in the MapieQuantileRegressor class to improve readability, modularity, and restructure it so that the function can be reused in Mapie v1.

Refactor Changes in fit

1. _initialize_fit_conformalize

   • Initializes key attributes like cv, alpha_np, and estimators_.
   • Ensures parameters are validated before proceeding.

2. _prepare_train_calib

   • Prepares training and calibration datasets.
   • Handles validation, splitting, and formatting of inputs (X, y, sample_weight, etc.).

3. _fit_estimators

   • Fits estimators for the specified quantile values (alpha).
   • Stores trained estimators in the estimators_ attribute.

4. conformalize

   • Computes conformity scores based on calibration data.
   • Handles residual calculations and stores them for prediction intervals.
     .

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• I have read the contributing guidelines
• I have updated the HISTORY.rst and AUTHORS.rst files
• Linting passes successfully : make lint
• Typing passes successfully : make type-check
• Unit tests pass successfully : make tests
• Coverage is 100% : make coverage
• Documentation builds successfully and without warnings : make doc