* Disallow importing non-dask estimators from xgboost.dask
This is mostly a style change, but also avoids a user error (that I have
committed on a few occasions). Since `XGBRegressor` and `XGBClassifier`
are imported as parent classes for the `dask` estimators, without
defining an `__all__`, autocomplete (or muscle) memory will produce the
following with little prompting:
```
from xgboost.dask import XGBClassifier
```
There's nothing inherently wrong with that, but given that
`XGBClassifier` is not `dask` enabled, it can lead to confusing behavior
until you figure out you should've typed
```
from xgboost.dask import DaskXGBClassifier
```
Another option is to alias import the existing non-dask estimators.
* Remove base/iter class, add train predict funcs
* Use type aliases for discard iterators
* update to include host_vector as thrust 1.12 doesn't bring it in as a side-effect
* cub::DispatchRadixSort requires signed offset types
- Reduce dependency on dmlc parsers and provide an interface for users to load data by themselves.
- Remove use of threaded iterator and IO queue.
- Remove `page_size`.
- Make sure the number of pages in memory is bounded.
- Make sure the cache can not be violated.
- Provide an interface for internal algorithms to process data asynchronously.
The role of ProxyDMatrix is going beyond what it was designed. Now it's used by both
QuantileDeviceDMatrix and inplace prediction. After the refactoring of sparse DMatrix it
will also be used for external memory. Renaming the C API to extract it from
QuantileDeviceDMatrix.
Other than modularizing the split evaluation function, this PR also removes some more functions including `InitNewNodes` and `BuildNodeStats` among some other unused variables. Also, scattered code like setting leaf weights is grouped into the split evaluator and `NodeEntry` is simplified and made private. Another subtle difference with the original implementation is that the modified code doesn't call `tree[nidx].Parent()` to traversal upward.
* Add feature score support for linear model.
* Port R interface to the new implementation.
* Add linear model support in Python.
Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>
* Support categorical data for dask functional interface and DQM.
* Implement categorical data support for GPU GK-merge.
* Add support for dask functional interface.
* Add support for DQM.
* Get newer cupy.
