### What changes were proposed in this pull request?
Add ```__repr__``` in Python ML Models
### Why are the changes needed?
In Python ML Models, some of them have ```__repr__```, others don't. In the doctest, when calling Model.setXXX, some of the Models print out the xxxModel... correctly, some of them can't because of lacking the ```__repr__``` method. For example:
```
>>> gm = GaussianMixture(k=3, tol=0.0001, seed=10)
>>> model = gm.fit(df)
>>> model.setPredictionCol("newPrediction")
GaussianMixture...
```
After the change, the above code will become the following:
```
>>> gm = GaussianMixture(k=3, tol=0.0001, seed=10)
>>> model = gm.fit(df)
>>> model.setPredictionCol("newPrediction")
GaussianMixtureModel...
```
### Does this PR introduce any user-facing change?
Yes.
### How was this patch tested?
doctest
Closes#26489 from huaxingao/spark-29876.
Authored-by: Huaxin Gao <huaxing@us.ibm.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
### 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>
