text: Title: r2 metric report values
  Body: When using r2 as eval metric for regression task (with 'Explain' mode) the metric values reported in Leaderboard (at README.md file) are multiplied by -1.
  For instance, the metric value for some model shown in the Leaderboard is -0.41, while when clicking the model name leads to the detailed results page - and there the value of r2 is 0.41.
  I've noticed that when one of R2 metric values in the Leaderboard was > 1 (~ 45), which shouldn't be possible for R2 metric, then I looked in the detailed results and saw that the value is actually ~ -45.
  