Hey,
Thank you for all this information.
If you want to do this to learn how to use SageMaker I definitely suggest building an NLP model and then integrate it afterwards into your multimodal data.
If you still want to use the multimodal-transformers you could create a requirements.txt in the same directory as your entry_point SageMaker will then install these requirements before running your training.
And for your inference, you can take a look here at how to build custom inference.py Deploy models to Amazon SageMaker 3