Dice helper enhancement

[wyli]

One small comment here

MONAI/monai/metrics/meandice.py

Lines 206 to 208 in baa17a8

	if denorm <= 0:
	return torch.tensor(1.0, device=y_o.device)
	return (2.0 * torch.sum(torch.masked_select(y, y_pred))) / denorm

if we reach the last "return" statement, it seems to be guaranteed to be 0, so we don't need to calculate it

Originally posted by @myron in #6163 (comment)

docstring example could be improved as well to use either sigmoid or softmax

MONAI/monai/metrics/meandice.py

Line 159 in baa17a8

score, not_nans = DiceHelper(include_background=False, sigmoid=True, softmax=True)(y_pred, y)