### What changes were proposed in this pull request?
add weight support for GBTs by sampling data before passing it to trees and then passing weights to trees
in summary:
1, add setters of `minWeightFractionPerNode` & `weightCol`
2, update input types in private methods from `RDD[LabeledPoint]` to `RDD[Instance]`:
`DecisionTreeRegressor.train`, `GradientBoostedTrees.run`, `GradientBoostedTrees.runWithValidation`, `GradientBoostedTrees.computeInitialPredictionAndError`, `GradientBoostedTrees.computeError`,
`GradientBoostedTrees.evaluateEachIteration`, `GradientBoostedTrees.boost`, `GradientBoostedTrees.updatePredictionError`
3, add new private method `GradientBoostedTrees.computeError(data, predError)` to compute average error, since original `predError.values.mean()` do not take weights into account.
4, add new tests
### Why are the changes needed?
GBTs should support sample weights like other algs
### Does this PR introduce any user-facing change?
yes, new setters are added
### How was this patch tested?
existing & added testsuites
Closes#25926 from zhengruifeng/gbt_add_weight.
Authored-by: zhengruifeng <ruifengz@foxmail.com>
Signed-off-by: zhengruifeng <ruifengz@foxmail.com>
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