code: def transform(self, X, lenscale=None):
          r
          N, d = X.shape
          lenscale = self._check_dim(d, lenscale)
  
          return expit(cdist(X / lenscale, self.C / lenscale, 'euclidean'))
docs: r"""
          Apply the sigmoid basis function to X.
  
          Parameters
          ----------
          X: ndarray
              (N, d) array of observations where N is the number of samples, and
              d is the dimensionality of X.
          lenscale: float
              the length scale (scalar) of the RBFs to apply to X. If not input,
              this uses the value of the initial length scale.
  
          Returns
          -------
          ndarray:
              of shape (N, D) where D is number of centres.