--gpus flag with add_argparse_args bug
🐛 Bug
When using
parser = Trainer.add_argparse_args(parser)
args = parser.parse_args()

trainer = Trainer.from_argparse_args(args)

if the user does not provide the --gpus flag, the code allocates some memory on the GPU and reports
GPU available: True, used: True
but does not use this GPU. This issue has been discovered in bolts:

PyTorchLightning/lightning-bolts#35
PyTorchLightning/lightning-bolts#124

To Reproduce
Steps to reproduce the behavior:
Use the following method to create the trainer object
parser = Trainer.add_argparse_args(parser)
args = parser.parse_args()

trainer = Trainer.from_argparse_args(args)

and don't pass the --gpus flag.
Expected behavior
If --gpus flag is not provided in the script call, then Lightning should report
GPU available: True, used: False
with a warning, and not allocate any memory on the GPU. This way, the user can set the --gpus flag if they have missed out on it.