cli: Confused on (str, int, List[int]) variants for argparse for --gpus flag?

🐛 Bug

A colleague (@siyuanfeng-tri) and I sometimes get confused on how the --gpus flag is to be interpreted by argparse. I see the following docs: https://pytorch-lightning.readthedocs.io/en/1.2.1/advanced/multi_gpu.html#select-gpu-devices

But we’re sometimes confused about when argparse interpretation will either assume it’s the count of the gpu (int/str) or the device index (List[int]).

Are there docs for this? If not, can that be clarified somehow?

Please reproduce using the BoringModel

see notebook

The main complaint is that gpus=3 implies gpus=[3], while gpus="3" implies gpus=[0,1,2]. Mix that with implicit conversion from argparse from str to int, and you get a kinda weird public interface.

To Reproduce

example notebook: https://colab.research.google.com/drive/1pe9_F2S73-gQ3hOeh_MMiGhmbXmGURDQ?usp=sharing

Expected behavior

Less confusing / more explicit options? (maybe my complaint is with weird implicit behavior of Trainer(gpus=...)?)

Environment

• PyTorch Version (e.g., 1.7.1):
• OS: Ubuntu 18.04
• How you installed PyTorch: pip
• Python version: 3.6.9
• CUDA/cuDNN version: N/A
• GPU models and configuration: N/A
• Any other relevant information: N/A

Additional context

N/A

[williamFalcon]

i vote for making it consistent!

[edgarriba]

vote for @awaelchli proposal - got some confusion in that too some time ago.