Fix num_channels_per_class=1 behavior in structure_model_output

[Copilot]

The structure_model_output function documented that num_channels_per_class only applies when > 1, but treated 1 as "split into per-class dict" instead of returning a tensor like None does.

Changes

• Modified conditional checks: Changed if num_channels_per_class is not None: to if num_channels_per_class is not None and num_channels_per_class > 1: in both dict-input and tensor-input branches
• Updated docstring: Clarified that both None and 1 mean one channel per class
• Added test coverage: Two test cases verifying num_channels_per_class=1 returns tensors, not dicts

Before/After

# Before: num_channels_per_class=1 incorrectly split into dict
outputs = torch.zeros(2, 3, 8, 8)  # 3 classes, 1 ch each
result = structure_model_output(outputs, classes, num_channels_per_class=1)
# Returns: {"output": {"class1": tensor, "class2": tensor, "class3": tensor}}

# After: num_channels_per_class=1 behaves like None
result = structure_model_output(outputs, classes, num_channels_per_class=1)
# Returns: {"output": tensor}  # Same as num_channels_per_class=None

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