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