code: def from_6x1_to_3x3(V):
      """Convert 6 x 1 vectors into symmetric 3 x 3 matrices.
  
      Parameters
      ----------
      V : numpy.ndarray
          An array of size (..., 6, 1).
  
      Returns
      -------
      T : numpy.ndarray
          Converted matrices of size (..., 3, 3).
  
      Notes
      -----
      The conversion of a matrix into a vector is defined as
  
          .. math::
  
              \mathbf{V} = \begin{bmatrix}
              T_{11} & T_{22} & T_{33} &
              \sqrt{2} T_{23} & \sqrt{2} T_{13} & \sqrt{2} T_{12}
              \end{bmatrix}^T
      """
      if V.shape[-2::] != (6, 1):
          raise ValueError(
              "The shape of the input array must be (..., 6, 1)."
          )
      C = 1 / np.sqrt(2)
      T = np.array(
          (
              [V[..., 0, 0], C * V[..., 5, 0], C * V[..., 4, 0]],
              [C * V[..., 5, 0], V[..., 1, 0], C * V[..., 3, 0]],
              [C * V[..., 4, 0], C * V[..., 3, 0], V[..., 2, 0]],
          )
      )
      T = np.moveaxis(T, (0, 1), (-2, -1))
      return T
  
imports: from warnings import warn
  import numpy
  from dipy.reconst.base import ReconstModel
  from dipy.reconst.dti import auto_attr
  from dipy.utils.optpkg import optional_package
label_desc: Use image node_id 2 for calling a global function with example usage: from_6x1_to_3x3(V) and returns: T