code: def get_symmetric_wallace_tensor(self, tau):
          
          wallace = self.get_wallace_tensor(tau)
          return Tensor(0.5 * (wallace + np.transpose(wallace, [2, 3, 0, 1])))
docs: Gets the symmetrized wallace tensor for determining
          yield strength criteria.
  
          Args:
              tau (3x3 array-like): stress at which to evaluate
                  the wallace tensor.