### What changes were proposed in this pull request?
Follow Scala ml tuning implementation
- put leading underscore before python ```ValidatorParams``` to indicate private
- add ```_CrossValidatorParams``` and ```_TrainValidationSplitParams```
- separate the getters and setters. Put getters in _XXXParams and setters in the Classes.
### Why are the changes needed?
Keep parity between scala and python
### Does this PR introduce any user-facing change?
add ```CrossValidatorModel.getNumFolds``` and ```TrainValidationSplitModel.getTrainRatio()```
### How was this patch tested?
Add doctest
Closes#26057 from huaxingao/spark-tuning.
Authored-by: Huaxin Gao <huaxing@us.ibm.com>
Signed-off-by: zhengruifeng <ruifengz@foxmail.com>