Let $A = \left[ {\begin{array}{*{20}{c}}
{\cos \theta }&0&{ - \sin \theta }\\
0&1&0\\
{\sin \theta }&0&{\cos \theta }
\end{array}} \right]$. If for some $\theta \in (0,\pi ),{A^2} = {A^T}$, then the sum of the diagonal elements of the matrix ${({\rm{A}} + {\rm{I}})^3} + {({\rm{A}} - {\rm{I}})^3} - 6\;{\rm{A}}$ is equal to __________.
