Andy Adinets 42bf90eb8f Column sampling at individual nodes (splits). (#3971)
* Column sampling at individual nodes (splits).

* Documented colsample_bynode parameter.

- also updated documentation for colsample_by* parameters

* Updated documentation.

* GetFeatureSet() returns shared pointer to std::vector.

* Sync sampled columns across multiple processes.
2018-12-14 22:37:35 +08:00
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