### What changes were proposed in this pull request?
- Move tree related classes to a separate file ```tree.py```
- add method ```predictLeaf``` in ```DecisionTreeModel```& ```TreeEnsembleModel```
### Why are the changes needed?
- keep parity between scala and python
- easy code maintenance
### Does this PR introduce any user-facing change?
Yes
add method ```predictLeaf``` in ```DecisionTreeModel```& ```TreeEnsembleModel```
add ```setMinWeightFractionPerNode``` in ```DecisionTreeClassifier``` and ```DecisionTreeRegressor```
### How was this patch tested?
add some doc tests
Closes#25929 from huaxingao/spark_29116.
Authored-by: Huaxin Gao <huaxing@us.ibm.com>
Signed-off-by: zhengruifeng <ruifengz@foxmail.com>
