Environment
monai 1.0.1

Describe the bug
When using include_background=False, softmax=True in DiceFocalLoss for binary segmentation, the Dice part produces this warning and does not apply softmax when calculating the Dice loss:

UserWarning: single channel prediction, `softmax=True` ignored.
  warnings.warn("single channel prediction, `softmax=True` ignored.")

To Reproduce

import torch
from monai.losses import DiceFocalLoss
from monai.networks import one_hot

tx = torch.rand(1, 2, 256, 256) # logits
ty = one_hot((torch.rand(1, 1, 256, 256) > 0.5).long(), 2) # one-hot label
loss = DiceFocalLoss(include_background=False, softmax=True)
loss(tx, ty)

Expected behavior & Additional context
By include_background=False, since the first channel is removed before passing the input and target to Dice and Focal losses, the Dice loss cannot apply softmax to the input which now has only one channel.