In MONAI's focal loss implementation, why does the alpha parameter become invalid when softmax is used and include_background=False?

Discussed in https://github.com/Project-MONAI/MONAI/discussions/8116

^{Originally posted by dancingreindeer September 26, 2024}

if self.use_softmax: 
   if not self.include_background and self.alpha is not None: 
            self.alpha = None 
            warnings.warn("include_background=False, alpha ignored when using softmax.") 
        loss = softmax_focal_loss(input, target, self.gamma, self.alpha)

This is the code piece. Not sure why when we don't include background and when using softmax, alpha became invalid? Does this assume there are no label imbalance? How to address this when there is significantly more negative samples than positive samples in the experiment then?

Start with the focal loss implementation's softmax branch shown in the issue and read the linked discussion #8116. Determine why alpha is cleared when include_background=False, whether this reflects a limitation or intended behavior, and document how class imbalance should be handled in this configuration.