Describe the bug
I'm using os.environ["CUDA_VISIBLE_DEVICES"] = '0' to set available GPU instead of specifing device like cuda:0.
Everthing works fine until updating to ver. 0.5.0. Maybe some GPU-related operations were added in the latest version during import phase.

To Reproduce
Steps to reproduce the behavior:

1. Use one tutorial for example.
2. Default behavior: All GPUs are detected and only one gpu is actually used
3. Add os.environ["CUDA_VISIBLE_DEVICES"] = '0' after def main(tempdir): on line 31.

• ver. 0.4.x : Only GPU-0 is detected and used.
• ver. 0.5.0 : All GPUs are detected.

Environment
MONAI version: 0.5.0
Numpy version: 1.19.4
Pytorch version: 1.6.0
MONAI flags: HAS_EXT = False, USE_COMPILED = False
MONAI rev id: 2707407

Optional dependencies: Pytorch Ignite version: 0.4.4
Nibabel version: 3.2.1
scikit-image version: 0.17.2
Pillow version: 8.0.1
Tensorboard version: 2.5.0a20201221
gdown version: NOT INSTALLED or UNKNOWN VERSION. TorchVision version: 0.7.0
ITK version: 5.1.2
tqdm version: 4.54.1
lmdb version: NOT INSTALLED or UNKNOWN VERSION.
psutil version: 5.7.3