Get Dice for each class in `DiceMetric`

Now when reduction="mean_batch", will return the dice metrics as a tensor without label names.
https://github.com/Project-MONAI/MONAI/blob/cc20c9bee91379e10762a15dd0f48c3e78271eb3/monai/metrics/meandice.py#L114