Release 0.12.0

Breaking changes:

• Class weights are now taken into account by eval_metrics(), get_feature_importance(), and get_object_importance().
  In previous versions the weights were ignored.
• Parameter random-strength for pairwise training (PairLogitPairwise, QueryCrossEntropy, YetiRankPairwise) is not supported anymore.
• Simultaneous use of MultiClass and MultiClassOneVsAll metrics is now deprecated.

New functionality:

• cv method is now supported on GPU.
• String labels for classes are supported in Python.
  In multiclassification the string class names are inferred from the data.
  In binary classification for using string labels you should employ class_names parameter and specify which class is negative (0) and which is positive (1).
  You can also use class_names in multiclassification mode to pass all possible class names to the fit function.
• Borders can now be saved and reused.
  To save the feature quantization information obtained during training data preprocessing into a text file use cli option --output-borders-file.
  To use the borders for training use cli option --input-borders-file.
  This functionanlity is now supported on CPU and GPU (it was GPU-only in previous versions).
  File format for the borders is described here.
• CLI option --eval-file is now supported on GPU.

Quality improvement:

• Some cases in binary classification are fixed where training could diverge

Optimizations:

• A great speedup of the Python applier (10x)
• Reduced memory consumption in Python cv function (times fold count)

Benchmarks and tutorials:

• Added speed benchmarks for CPU and GPU on a variety of different datasets.
• Added benchmarks of different ranking modes. In this tutorial we compare different ranking modes in CatBoost, XGBoost and LightGBM.
• Added tutorial for applying model in Java.
• Added benchmarks of SHAP values calculation for CatBoost, XGBoost and LightGBM.
  The benchmarks also contain explanation of complexity of this calculation in all the libraries.

We also made a list of stability improvements and stricter checks of input data and parameters.

And we are so grateful to our community members @canorbal and @neer201 for their contribution to this release. Thank you.