*   If an RNNCell is used twice in two different variable scopes, an error
        is raised describing how to avoid this behavior.
    *   If an RNNCell is used in a variable scope with existing conflicting
        variables, an error is raised showing that the RNNCell must be
        constructed with argument `reuse=True`.
*   Deprecated contrib/distributions `pmf`, `pdf`, `log_pmf`, `log_pdf`.