Wrong channel in `DiceHelper`

[KumoLiu]

Describe the bug
DiceHelper should add a n_classs arg if the user adds post-processing on the prediction, L218 will generate wrong channel.

MONAI/monai/metrics/meandice.py

Line 218 in fbf8847

n_pred_ch = y_pred.shape[1]

To Reproduce

import torch
import monai.transforms as mt
from monai.metrics import DiceHelper, DiceMetric

n_classes = 5
spatial_shape = (128, 128, 128)

y_pred = torch.rand(n_classes, *spatial_shape).float()  # predictions
y = torch.randint(0, n_classes, size=(1, *spatial_shape)).long()  # ground truth

y_pred_argmax = mt.AsDiscrete(argmax=True)(y_pred)

score, not_nans = DiceHelper(include_background=False, reduction="mean")(y_pred.unsqueeze(0), y.unsqueeze(0))
metric = DiceMetric(include_background=False)
metric([y_pred_argmax], [y])
_metric = metric.aggregate().item()
metric.reset()
print(_metric)
print(score, not_nans)

Expected behavior
add n_class in DiceHelper and DiceMetric
cc @wyli

[wyli]

previously the docstring states that y_pred should have n-channels though

MONAI/monai/metrics/meandice.py

Line 30 in 8279a30

`y_pred` is expected to have binarized predictions and `y` can be single-channel class indices or in the

MONAI/monai/metrics/meandice.py

Line 144 in 8279a30

`y_pred` must have N channels, `y` can be single-channel class indices or in the one-hot format.

so considering this as a feature request instead of a bug...