v1.6.0

Breaking changes

• The dependency to the distributed gradient boosted trees learner is renamed
  from
  //third_party/yggdrasil_decision_forests/learner/distributed_gradient_boosted_trees
  to
  //third_party/yggdrasil_decision_forests/learner/distributed_gradient_boosted_trees:dgbt.
  Note most case, importing the learners with
  //third_party/yggdrasil_decision_forests/learner:all_learners is
  recommended.
• The training configuration must contain a label. A missing label is no
  longer interpreted as the label being the input feature "".

Feature

• Add support for monotonic constraints for gradient boosted trees.
• Improve speed of dataset reading and writing.

Fix

• Proper error message when using distributed training on more than 2^31
  (i.e., ~2B) examples while compiling YDF with 32-bits example index.
• Fix Window compilation with Visual Studio 2019
• Improved error messages for invalid training configuration
• Replaced outdated dependencies