Added type hints for TF: TransfoXL
Based on Issue #16059 
I have added type hints for Tensorflow TransfoXL Model.


_The documentation is not available anymore as the PR was closed or merged._<|||||>I removed these Optional types as your suggested.
Holp you may check it and then merge my request.
@Rocketknight1 <|||||>Looks good to me, thank you!