code: def cv_nfolds_sd_check():
      prostate = h2o.import_file(
          path=pyunit_utils.locate("smalldata/logreg/prostate.csv")
      )
      prostate[1] = prostate[1].asfactor()
      prostate.summary()
  
      prostate_gbm = H2OGradientBoostingEstimator(
          nfolds=4, distribution="bernoulli"
      )
      prostate_gbm.train(
          x=list(range(2, 9)), y=1, training_frame=prostate
      )
      prostate_gbm.show()
      prostate_gbm.model_performance(xval=True)
      # extract mean and std column calculated by cross-validation metric
      meanCol = pyunit_utils.extract_col_value_H2OTwoDimTable(
          prostate_gbm._model_json["output"][
              "cross_validation_metrics_summary"
          ],
          "mean",
      )
      stdCol = pyunit_utils.extract_col_value_H2OTwoDimTable(
          prostate_gbm._model_json["output"][
              "cross_validation_metrics_summary"
          ],
          "sd",
      )
      # extract actual values from all folds
      cv1 = pyunit_utils.extract_col_value_H2OTwoDimTable(
          prostate_gbm._model_json["output"][
              "cross_validation_metrics_summary"
          ],
          "cv_1_valid",
      )
      cv2 = pyunit_utils.extract_col_value_H2OTwoDimTable(
          prostate_gbm._model_json["output"][
              "cross_validation_metrics_summary"
          ],
          "cv_2_valid",
      )
      cv3 = pyunit_utils.extract_col_value_H2OTwoDimTable(
          prostate_gbm._model_json["output"][
              "cross_validation_metrics_summary"
          ],
          "cv_3_valid",
      )
      cv4 = pyunit_utils.extract_col_value_H2OTwoDimTable(
          prostate_gbm._model_json["output"][
              "cross_validation_metrics_summary"
          ],
          "cv_4_valid",
      )
      cvVals = [cv1, cv2, cv3, cv4]
      assertMeanSDCalculation(meanCol, stdCol, cvVals)
  
imports: import sys
  import h2o
  from tests import pyunit_utils
  from h2o.estimators.gbm import H2OGradientBoostingEstimator
  import math
label_desc: Use image node_id 1 for calling a global function with example usage: cv_nfolds_sd_check() without return types