code: def from_21x1_to_6x6(V):
      """Convert 21 x 1 vectors into symmetric 6 x 6 matrices.
  
      Parameters
      ----------
      V : numpy.ndarray
          An array of size (..., 21, 1).
  
      Returns
      -------
      T : numpy.ndarray
          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[-2::] != (21, 1):
          raise ValueError(
              "The shape of the input array must be (..., 21, 1)."
          )
      C = 1 / np.sqrt(2)
      T = np.array(
          (
              [
                  V[..., 0, 0],
                  C * V[..., 5, 0],
                  C * V[..., 4, 0],
                  C * V[..., 6, 0],
                  C * V[..., 7, 0],
                  C * V[..., 8, 0],
              ],
              [
                  C * V[..., 5, 0],
                  V[..., 1, 0],
                  C * V[..., 3, 0],
                  C * V[..., 9, 0],
                  C * V[..., 10, 0],
                  C * V[..., 11, 0],
              ],
              [
                  C * V[..., 4, 0],
                  C * V[..., 3, 0],
                  V[..., 2, 0],
                  C * V[..., 12, 0],
                  C * V[..., 13, 0],
                  C * V[..., 14, 0],
              ],
              [
                  C * V[..., 6, 0],
                  C * V[..., 9, 0],
                  C * V[..., 12, 0],
                  V[..., 15, 0],
                  C * V[..., 18, 0],
                  C * V[..., 20, 0],
              ],
              [
                  C * V[..., 7, 0],
                  C * V[..., 10, 0],
                  C * V[..., 13, 0],
                  C * V[..., 18, 0],
                  V[..., 16, 0],
                  C * V[..., 19, 0],
              ],
              [
                  C * V[..., 8, 0],
                  C * V[..., 11, 0],
                  C * V[..., 14, 0],
                  C * V[..., 20, 0],
                  C * V[..., 19, 0],
                  V[..., 17, 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 4 for calling a global function with example usage: from_21x1_to_6x6(V) and returns: T