Trainer.on_gpu incorrectly set to False when specifying `gpus=0`
🐛 Bug
When creating a trainer with the arg gpus=0, the field on_gpu is always set False, even on machines with CUDA available.
The existing logic for on_gpu is:
self.on_gpu = True if (gpus and torch.cuda.is_available()) else False

is buggy because 0 is "falsy". It should probably be:
self.on_gpu = gpus is not None and torch.cuda.is_available()

To Reproduce
trainer = trainer.Trainer(gpus=0, ...)