docstr: Generalized cross-validation score of current fit.
  
  Craven, P. and Wahba, G.  "Smoothing noisy data with spline functions.
  Estimating the correct degree of smoothing by
  the method of generalized cross-validation."
  Numerische Mathematik, 31(4), 377-403.
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code: def gcv(self):
      """
      Generalized cross-validation score of current fit.
  
      Craven, P. and Wahba, G.  "Smoothing noisy data with spline functions.
      Estimating the correct degree of smoothing by
      the method of generalized cross-validation."
      Numerische Mathematik, 31(4), 377-403.
      """
  
      norm_resid = (self.resid**2).sum()
      return norm_resid / (self.df_total - self.trace())
  
imports: import numpy
  import numpy.linalg
  from scipy.linalg import solveh_banded
  from scipy.optimize import golden
  from models import _hbspline
  import warnings
label_desc: Use image node_id 3 for calling the SmoothingSpline obj's underlying member method code with example usage: obj.gcv() and returns: unknown