class_docstr: Builds Approximation instance from a given trace,
  it has the same interface as variational approximation
class_docstr_tok: ["Builds","Approximation","instance","from","a","given","trace",",","it","has","the","same","interface","as","variational","approximation"]
code: def cov(self):
      x = self.histogram - self.mean
      return x.T.dot(x) / pm.floatX(self.histogram.shape[0])
  
imports: from typing import Optional
  import numpy
  import pytensor
  from arviz import InferenceData
  from pytensor import tensor
  from pytensor.graph.basic import Variable
  from pytensor.graph.replace import graph_replace
  from pytensor.tensor.variable import TensorVariable
  import pymc
  from pymc.blocking import DictToArrayBijection
  from pymc.distributions.dist_math import rho2sigma
  from pymc.util import makeiter
  from pymc.variational import opvi
  from pymc.variational.opvi import Approximation, Group, NotImplementedInference, _known_scan_ignored_inputs, node_property
label_desc: Use image node_id 9 for calling the EmpiricalGroup obj's underlying member method code with example usage: obj.cov() and returns: unknown