Multi-GPU Training - Two Suggested Improvements

[RylanSchaeffer]

Hi! I'm not sure whether this is appropriate to open as an issue, but I can't find answers to two key multi-GPU training questions:

1. Does one need to use Trainer in order to take advantage of PL's multi-GPU training? Or is the trainer unnecessary if the models are LightningModules?

2. If I look at nvidia-smi and see that my PL code isn't using GPUs, how do I identify the cause?

cc @Borda