code: def __getitem__(self, index):
      model_params = self.model_params
      model_S0 = self.model_S0
      N = model_params.ndim
      if type(index) is not tuple:
          index = (index,)
      elif len(index) >= model_params.ndim:
          raise IndexError("IndexError: invalid index")
      index = index + (slice(None),) * (N - len(index))
      if model_S0 is not None:
          model_S0 = model_S0[index[:-1]]
      return MeanDiffusionKurtosisFit(
          self.model, model_params[index], model_S0=model_S0
      )
  
imports: import numpy
  import scipy.optimize
  from dipy.core.gradients import check_multi_b, unique_bvals_magnitude, round_bvals
  from dipy.reconst.base import ReconstModel
  from dipy.reconst.dti import MIN_POSITIVE_SIGNAL
  from dipy.core.ndindex import ndindex
  from dipy.core.onetime import auto_attr
label_desc: Use image node_id 2 for calling the MeanDiffusionKurtosisFit obj's underlying member method code with example usage: obj.__getitem__(index) and returns: MeanDiffusionKurtosisFit