[python package] include dmlc-tracker into xgb python pkg (#4731)

python-package/xgboost/__init__.py

@@ -12,6 +12,8 @@ from .core import DMatrix, Booster
 from .training import train, cv
 from . import rabit                   # noqa
 from . import dask  # noqa
+from . import tracker  # noqa
+from .tracker import RabitTracker  # noqa
 try:
     from .sklearn import XGBModel, XGBClassifier, XGBRegressor, XGBRanker
     from .sklearn import XGBRFClassifier, XGBRFRegressor
@@ -25,6 +27,7 @@ with open(VERSION_FILE) as f:
 
 __all__ = ['DMatrix', 'Booster',
            'train', 'cv',
+           'RabitTracker',
            'XGBModel', 'XGBClassifier', 'XGBRegressor', 'XGBRanker',
            'XGBRFClassifier', 'XGBRFRegressor',
            'plot_importance', 'plot_tree', 'to_graphviz']