[dask] Add DaskXGBRanker (#6576)

* Initial support for distributed LTR using dask.

* Support `qid` in libxgboost.
* Refactor `predict` and `n_features_in_`, `best_[score/iteration/ntree_limit]`
  to avoid duplicated code.
* Define `DaskXGBRanker`.

The dask ranker doesn't support group structure, instead it uses query id and
convert to group ptr internally.

include/xgboost/learner.h

@@ -165,7 +165,7 @@ class Learner : public Model, public Configurable, public dmlc::Serializable {
    * \brief Get the number of features of the booster.
    * \return number of features
    */
-  virtual uint32_t GetNumFeature() = 0;
+  virtual uint32_t GetNumFeature() const = 0;
 
   /*!
    * \brief Set additional attribute to the Booster.