code: def cvxpy_1x21_to_6x6(V):
      """Convert 1 x 21 vector into a symmetric 6 x 6 matrix.
  
      Parameters
      ----------
      V : numpy.ndarray
          An array of size (1, 21).
  
      Returns
      -------
      T : cvxpy.bmat
          Converted matrices of size (6, 6).
  
      Notes
      -----
      The conversion of a matrix into a vector is defined as
  
          .. math::
  
              \begin{matrix}
              \mathbf{V} = & \big[
              T_{11} & T_{22} & T_{33} \\
              & \sqrt{2} T_{23} & \sqrt{2} T_{13} & \sqrt{2} T_{12} \\
              & \sqrt{2} T_{14} & \sqrt{2} T_{15} & \sqrt{2} T_{16} \\
              & \sqrt{2} T_{24} & \sqrt{2} T_{25} & \sqrt{2} T_{26} \\
              & \sqrt{2} T_{34} & \sqrt{2} T_{35} & \sqrt{2} T_{36} \\
              & T_{44} & T_{55} & T_{66} \\
              & \sqrt{2} T_{45} & \sqrt{2} T_{56} & \sqrt{2} T_{46} \big]^T
              \end{matrix}
      """
      if V.shape[0] == 21:
          V = V.T
  
      f = 1 / np.sqrt(2)
  
      T = cp.bmat(
          [
              [
                  V[0, 0],
                  f * V[0, 5],
                  f * V[0, 4],
                  f * V[0, 6],
                  f * V[0, 7],
                  f * V[0, 8],
              ],
              [
                  f * V[0, 5],
                  V[0, 1],
                  f * V[0, 3],
                  f * V[0, 9],
                  f * V[0, 10],
                  f * V[0, 11],
              ],
              [
                  f * V[0, 4],
                  f * V[0, 3],
                  V[0, 2],
                  f * V[0, 12],
                  f * V[0, 13],
                  f * V[0, 14],
              ],
              [
                  f * V[0, 6],
                  f * V[0, 9],
                  f * V[0, 12],
                  V[0, 15],
                  f * V[0, 18],
                  f * V[0, 20],
              ],
              [
                  f * V[0, 7],
                  f * V[0, 10],
                  f * V[0, 13],
                  f * V[0, 18],
                  V[0, 16],
                  f * V[0, 19],
              ],
              [
                  f * V[0, 8],
                  f * V[0, 11],
                  f * V[0, 14],
                  f * V[0, 20],
                  f * V[0, 19],
                  V[0, 17],
              ],
          ]
      )
      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 6 for calling a global function with example usage: cvxpy_1x21_to_6x6(V) and returns: T