fix set visible devices

[rijobro]

Fixes #2161.

Description

In TestTimeAugmentation, I called torch.cuda as a default argument to a constructor. This meant that the following code previously worked and now didn't:

import os
import monai
os.environ["CUDA_VISIBLE_DEVICES"] = '0'

I fixed the problem and also created a unit test to avoid it happening in the future. The trick is that we shouldn't call torch.cuda. during import monai.

Status

Ready

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[Nic-Ma]

To solve the flake8 issue, maybe you can default to a str "cpu", like what we did for MONAI metrics:
https://github.com/Project-MONAI/MONAI/blob/dev/monai/handlers/iteration_metric.py#L51

Thanks.