[dask] Disallow importing non-dask estimators from xgboost.dask (#7133)

* 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

python-package/xgboost/dask.py

@@ -48,7 +48,6 @@ from .sklearn import xgboost_model_doc
 from .sklearn import _cls_predict_proba
 from .sklearn import XGBRanker
 
-
 if TYPE_CHECKING:
     from dask import dataframe as dd
     from dask import array as da
@@ -71,6 +70,20 @@ try:
 except ImportError:
     TrainReturnT = Dict[str, Any]  # type:ignore
 
+__all__ = [
+    "RabitContext",
+    "DaskDMatrix",
+    "DaskDeviceQuantileDMatrix",
+    "DaskXGBRegressor",
+    "DaskXGBClassifier",
+    "DaskXGBRanker",
+    "DaskXGBRFRegressor",
+    "DaskXGBRFClassifier",
+    "train",
+    "predict",
+    "inplace_predict",
+]
+
 # TODOs:
 #   - CV
 #