Fix: correct import, English docstring, safer channel heuristic; robust y channel-last detection (refs #8366)

Signed-off-by: 林旻佑 <[email protected]>

Author: 林旻佑

monai/metrics/meandice.py

@@ -15,7 +15,7 @@
 
 from monai.metrics.utils import do_metric_reduction
 from monai.utils import MetricReduction, deprecated_arg
-from monai.metrics.utils import ensure_channel_first
+from monai.inferers.utils import ensure_channel_first
 
 
 from .metric import CumulativeIterationMetric