Commits on Nov 26, 2014

1. [SPARK-4583] [mllib] LogLoss for GradientBoostedTrees fix + doc updates …

   Currently, the LogLoss used by GradientBoostedTrees has 2 issues:
   * the gradient (and therefore loss) does not match that used by Friedman (1999)
   * the error computation uses 0/1 accuracy, not log loss
   
   This PR updates LogLoss.
   It also adds some doc for boosting and forests.
   
   I tested it on sample data and made sure the log loss is monotonically decreasing with each boosting iteration.
   
   CC: mengxr manishamde codedeft
   
   Author: Joseph K. Bradley <joseph@databricks.com>
   
   Closes apache#3439 from jkbradley/gbt-loss-fix and squashes the following commits:
   
   cfec17e [Joseph K. Bradley] removed forgotten temp comments
   a27eb6d [Joseph K. Bradley] corrections to last log loss commit
   ed5da2c [Joseph K. Bradley] updated LogLoss (boosting) for numerical stability
   5e52bff [Joseph K. Bradley] * Removed the 1/2 from SquaredError.  This also required updating the test suite since it effectively doubles the gradient and loss. * Added doc for developers within RandomForest. * Small cleanup in test suite (generating data only once)
   e57897a [Joseph K. Bradley] Fixed LogLoss for GradientBoostedTrees, and updated doc for losses, forests, and boosting
   
   (cherry picked from commit c251fd7)
   Signed-off-by: Xiangrui Meng <meng@databricks.com>

   

   jkbradley authored and mengxr committed Nov 26, 2014
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2. add import o.a.s.SparkContext._ for v1.2

   

   mengxr committed Nov 26, 2014
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