Include option to set num_worker for PyTorch Lightning models

[JulianZabbarov]

Finding the optimal number of workers for the PyTorch Lightning models is key to optimize the training performance of models on local machines and cluster. Thus, it is important that user can manually select the num_worker for each model.

To-Do:

• Add option to manually set num_workers
• Set default to number of cpus of the current machine.

For best practices, see: https://lightning.ai/docs/pytorch/stable/advanced/speed.html