code: def from_3x3_to_6x1(T):
      """Convert symmetric 3 x 3 matrices into 6 x 1 vectors.
  
      Parameters
      ----------
      T : numpy.ndarray
          An array of size (..., 3, 3).
  
      Returns
      -------
      V : numpy.ndarray
          Converted vectors of size (..., 6, 1).
  
      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 T.shape[-2::] != (3, 3):
          raise ValueError(
              "The shape of the input array must be (..., 3, 3)."
          )
      if not np.all(np.isclose(T, np.swapaxes(T, -1, -2))):
          warn(
              "All matrices converted to Voigt notation are not symmetric."
          )
      C = np.sqrt(2)
      V = np.stack(
          (
              T[..., 0, 0],
              T[..., 1, 1],
              T[..., 2, 2],
              C * T[..., 1, 2],
              C * T[..., 0, 2],
              C * T[..., 0, 1],
          ),
          axis=-1,
      )[..., np.newaxis]
      return V
  
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 1 for calling a global function with example usage: from_3x3_to_6x1(T) and returns: V