v0.17.4

Improvements:

• Massive 2x speedup for MultiClass with many classes
• Updated MVS implementation. See Minimal Variance Sampling in Stochastic Gradient Boosting by Bulat Ibragimov and Gleb Gusev at NeurIPS 2019
• Added sum_models in R-package, #1007

Bugs fixed:

• Multi model initialization in python, #995
• Mishandling of 255 borders in training on GPU, #1010