docstr: We constraint the last three parameters
  ('var.e', 'var.x.coeff', 'var.w.coeff') to be positive,
  because they are variances
code: def transform_params(self, unconstrained):
      """
      We constraint the last three parameters
      ('var.e', 'var.x.coeff', 'var.w.coeff') to be positive,
      because they are variances
      """
      constrained = unconstrained.copy()
      constrained[self.positive_parameters] = (
          constrained[self.positive_parameters] ** 2
      )
      return constrained
  
imports: from collections import OrderedDict
  import matplotlib.pyplot
  import numpy
  import pandas
  import statsmodels.api
  import pymc3
  import theano
  import theano.tensor
label_desc: Use image node_id 4 for calling the TVRegressionExtended obj's underlying member method code with example usage: obj.transform_params(unconstrained) and returns: constrained