Add RankSEG post-transform tests Ver.2

[Leev1s]

Test strategy

tests/transforms/test_as_discrete.py and tests/transforms/test_as_discreted.py are part of MONAI's minimum test suite, but rankseg is an optional dependency installed in the full/development environment. The tests therefore separate minimum-safe API coverage from the real optional dependency path.

Minimum dependency tests still cover:

• existing AsDiscrete / AsDiscreted behavior;
• explicit rankseg=False with argmax=True;
• rankseg=True plus argmax=True incompatibility;
• missing optional dependency behavior via OptionalImportError.

Full/development tests additionally run a real RankSEG value/shape case. The chosen input is a two-class probability map.

img = [
    [[0.45, 0.55]],  # class 0
    [[0.55, 0.45]],  # class 1
]

For this input, argmax=True produces [[[1.0, 0.0]]], while the default RankSEG decoder produces [[1.0, 1.0]].

AsDiscreted full/development tests also cover dictionary forwarding, including the per-key rankseg=[True, False] case.

The real RankSEG cases are only added when the optional package is available, so minimum tests do not report dependency-related skips and continue to exercise the rest of the file.

Validation

Targeted tests passed locally:

uv run pytest tests/transforms/test_as_discrete.py tests/transforms/test_as_discreted.py
uv run python -m unittest tests.transforms.test_as_discrete tests.transforms.test_as_discreted

Results:

• pytest: 53 passed
• unittest: 53 passed