code: def _create_nested_dataframe(cases=5, dimensions=1, length=10):
      testy = pd.DataFrame(dtype=np.float32)
      for i in range(0, dimensions):
          instance_list = []
          for _ in range(0, cases):
              instance_list.append(pd.Series(np.random.randn(length)))
          testy["dimension_" + str(i + 1)] = instance_list
      return testy
  
imports: import pickle
  import numpy
  import pandas
  import pytest
  from sklearn.model_selection import KFold
  from sktime.classification.base import BaseClassifier
  from sktime.classification.deep_learning.base import BaseDeepClassifier
  from sktime.classification.distance_based import KNeighborsTimeSeriesClassifier
  from sktime.classification.dummy import DummyClassifier
  from sktime.utils._testing.estimator_checks import _assert_array_almost_equal
  from sktime.utils._testing.panel import _make_classification_y, _make_panel, make_classification_problem
  from sktime.utils.validation._dependencies import _check_soft_dependencies
label_desc: Use image node_id 6 for calling a global function with example usage: _create_nested_dataframe(cases, dimensions, length) and returns: testy