Update _toctree.yml (#18440)
Update _toctree.yml (#18440)

This PR moves GroupViT and LXMert to their correct sections. As pointed out by @NielsRogge and @LysandreJik, GroupViT and LXMert are both multimodal models.
huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers