LightGBM

Improving monotone constraints ("Fast" method; linked to #2305, #2717)  (#2770)

* Add util functions.

* Added monotone_constraints_method as a parameter.

* Add the intermediate constraining method.

* Updated tests.

* Minor fixes.

* Typo.

* Linting.

* Ran the parameter generator for the doc.

* Removed usage of the FeatureMonotone function.

* more fixes

* Fix.

* Remove duplicated code.

* Add debug checks.

* Typo.

* Bug fix.

* Disable the use of intermediate monotone constraints and feature sampling at the same time.

* Added an alias for monotone constraining method.

* Use the right variable to get the number of threads.

* Fix DEBUG checks.

* Add back check to determine if histogram is splittable.

* Added forgotten override keywords.

* Perform monotone constraint update only when necessary.

* Small refactor of FastLeafConstraints.

* Post rebase commit.

* Small refactor.

* Typo.

* Added comment and slightly improved logic of monotone constraints.

* Forgot a const.

* Vectors that are to be modified need to be pointers.

* Rename FastLeafConstraints to IntermediateLeafConstraints to match documentation.

* Remove overload of GoUpToFindLeavesToUpdate.

* Stop memory leaking.

* Fix cpplint issues.

* Fix checks.

* Fix more cpplint issues.

* Refactor config monotone constraints method.

* Typos.

* Remove useless empty lines.

* Add new line to separate includes.

* Replace unsigned ind by size_t.

* Reduce number of trials in tests to decrease CI time.

* Specify monotone constraints better in tests.

* Removed outer loop in test of monotone constraints.

* Added categorical features to the monotone constraints tests.

* Add blank line.

* Regenerate parameters automatically.

* Speed up ShouldKeepGoingLeftRight.

Co-authored-by: Charles Auguste <auguste@dubquantdev801.ire.susq.com>
Co-authored-by: guolinke <guolin.ke@outlook.com>