Missing `alpha` parameter of Focal loss in DiceFocalLoss

[dkalsan]

Describe the bug
The available class monai.losses.DiceFocalLoss is missing the alpha parameter for Focal loss. Is this a result of some design choice or just got overlooked in the implementation?

Expected behavior
alpha parameter should be passed to the monai.losses.FocalLoss or have the reason for its absence documented.

[KumoLiu]

Hi @dkalsan, what does alpha mean here, can you be more explicit? Thanks.

[dkalsan]

I mean the argument passed down to the FocalLoss class during construction, which results in an alpha-balanced Focal loss as per the paper. I believe this argument should also be available in the DiceFocalLoss since it's effectively just a wrapper around the two losses.

From the docstring of FocalLoss:

alpha – value of the alpha in the definition of the alpha-balanced Focal loss. The value should be in [0, 1]. Defaults to None.