how to convert multi distributed GPU to single GPU

I am working on a project about visual question answering that use distributed GPU and address of project is https://github.com/microsoft/TAP I want to convert multi-processing to single processing because my computer has only one GPU (01:00.0 VGA compatible controller: NVIDIA Corporation GP102 [GeForce GTX 1080 Ti] (rev a1)) that script in this project for create distributed pytorch and multi processing is https://github.com/microsoft/TAP/blob/main/pythia/utils/distributed_utils.py how to change source code to run this project on my computer