RF: fix stream bug causing performance regressions (#4644)

This nanoPR fixes performance regression caused due to improper stream assignments to the decision trees.


Before fix:

 | sno |  algo | input | cu_time | cpu_time | cuml_acc | cpu_acc | speedup | n_samples | n_features | max_depth | n_estimators | n_bins | n_streams | n_jobs | n_classes | 
 | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | 
 | 0 | RandomForestClassifier | numpy | 32.635321855545044 | 0.0 | 0.99468 | 0.0 | 0.0 | 800000 | 64 | 8 | 500 | 128 | 4 | -1 | 2 | 
 | 1 | RandomForestClassifier | numpy | 40.36453413963318 | 0.0 | 0.994855 | 0.0 | 0.0 | 800000 | 64 | 10 | 500 | 128 | 4 | -1 | 2 | 
 | 2 | RandomForestClassifier | numpy | 61.35148477554321 | 0.0 | 0.99504 | 0.0 | 0.0 | 800000 | 64 | 16 | 500 | 128 | 4 | -1 | 2 | 

After fix:

| sno | algo | input | cu_time | cpu_time | cuml_acc | cpu_acc | speedup | n_samples | n_features | max_depth | n_estimators | n_bins | n_streams | n_jobs | n_classes
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
0 | RandomForestClassifier | numpy | 28.637776374816895 | 0.0 | 0.99468 | 0.0 | 0.0 | 800000 | 64 | 8 | 500 | 128 | 4 | -1 | 2
1 | RandomForestClassifier | numpy | 34.11380743980408 | 0.0 | 0.994855 | 0.0 | 0.0 | 800000 | 64 | 10 | 500 | 128 | 4 | -1 | 2
2 | RandomForestClassifier | numpy | 47.153409481048584 | 0.0 | 0.99504 | 0.0 | 0.0 | 800000 | 64 | 16 | 500 | 128 | 4 | -1 | 2

Command run in `cuml/`
```
python python/cuml/run_benchmarks.py--num-rows 800000 --num-features 64 --skip-cpu --test-split 0.2 --cuml-param-sweep "n_bins=[128]" "n_streams=[4]" --cpu-param-sweep "n_jobs=[-1]" --param-sweep "max_depth=[8,10,16]" "n_estimators=[500]" --n-reps 1 --csv pool-2112-cls-800000.csv --dataset-param-sweep "n_classes=[2]" --dtype "fp32" --dataset classification -- RandomForestClassifier
```

Authors:
  - Venkat (https://github.com/venkywonka)

Approvers:
  - Dante Gama Dessavre (https://github.com/dantegd)

URL: #4644

cpp/src/randomforest/randomforest.cuh

@@ -161,7 +161,7 @@ class RandomForest {
 #pragma omp parallel for num_threads(n_streams)
     for (int i = 0; i < this->rf_params.n_trees; i++) {
       int stream_id = omp_get_thread_num();
-      auto s        = handle.get_stream_from_stream_pool(i);
+      auto s        = handle.get_stream_from_stream_pool(stream_id);
 
       this->get_row_sample(i, n_rows, &selected_rows[stream_id], s);