question: How does a gpu cluster system like SLRUM use ddp training ?
context: I need to use srun run python, so how does set Trainer of pl` correctly?
answer: Not sure what you are asking. Is the question how to use PL with SLURM?
  I can point you to the SLURM tutorial in our docs:
  https://pytorch-lightning.readthedocs.io/en/latest/clouds/slurm.html