[WIP] Add type hints for Lxmert (TF)
# What does this PR do?

This PR adds type hints to the Lxmert model for TensorFlow.

Models:

Lxmert: @LysandreJik

_The documentation is not available anymore as the PR was closed or merged._<|||||>@Rocketknight1 Is there anything else I need to add to this PR?<|||||>@elusenji No, sorry! I meant to merge it after the tests passed but lost track of it yesterday. Doing it now, and thanks again!