Fix: Small logic mistake in the `AsDiscrete.__call__` method

[dcfidalgo]

Hi MONAI Team! Thank you very much for this super nice framework, really appreciate it! Just found a small logic mistake in one of the transform classes. To reproduce:

import torch
from monai.transforms.post.array import AsDiscrete

transform = AsDiscrete(argmax=True)
prediction = torch.rand(2, 3, 3)

transform(prediction, argmax=False)
# will still apply argmax

Description

Proposed fix: argmax is explicitly checked for None in the __cal__ method.

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [ ] Breaking change (fix or new feature that would cause existing functionality to change).
• [ ] 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 --disttests.
• [ ] In-line docstrings updated.
• [ ] Documentation updated, tested make html command in the docs/ folder.

[KumoLiu]

/build