Put back LXMert example
# What does this PR do?

During the example reorganization, LXMert seems to have slipped into the cracks and got accidentally deleted. This PR puts it back.

Fixes #9309
Anyway, I doubt Lxmert is currently not supported for customized/ personal datasets (nlp+images, images are harder to prepare). See issues relevant to feature extraction in [Lxmert](https://github.com/airsplay/lxmert#faster-r-cnn-feature-extraction), for example, [issue#79](https://github.com/airsplay/lxmert/issues/79), [issue#86](https://github.com/airsplay/lxmert/issues/86).