fixes EnsureChannel dice_loss_metric_notes (#964)

Signed-off-by: Wenqi Li <wenqil@nvidia.com>

Fixes #963 

### Description
be specific with the channel dim 

### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [ ] Notebook runs automatically `./runner [-p <regex_pattern>]`

Signed-off-by: Wenqi Li <wenqil@nvidia.com>

Author: wyli

modules/dice_loss_metric_notes.ipynb

@@ -20,7 +20,7 @@
     "import torch\n",
     "from monai.losses import DiceLoss\n",
     "from monai.metrics import DiceMetric\n",
-    "from monai.transforms import EnsureChannelFirst, AsDiscrete, Compose\n",
+    "from monai.transforms import AsDiscrete, Compose\n",
     "\n",
     "\n",
     "def print_tensor(name, t):\n",
@@ -108,7 +108,7 @@
    "outputs": [],
    "source": [
     "# make one hot and add batch dimension\n",
-    "make_2_class = Compose([AsDiscrete(to_onehot=2), EnsureChannelFirst()])\n",
+    "make_2_class = Compose([AsDiscrete(to_onehot=2), lambda x: x[None]])\n",
     "\n",
     "grnd2 = make_2_class(grnd)\n",
     "pred2 = make_2_class(pred)"
@@ -305,7 +305,7 @@
    "outputs": [],
    "source": [
     "# make one hot and add batch dimension\n",
-    "make_3_class = Compose([AsDiscrete(to_onehot=3), EnsureChannelFirst()])\n",
+    "make_3_class = Compose([AsDiscrete(to_onehot=3), lambda x: x[None]])\n",
     "\n",
     "mgrnd2 = make_3_class(mgrnd)\n",
     "mpred2 = make_3_class(mpred)"
@@ -412,7 +412,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.8.13"
+   "version": "3.8.12"
   }
  },
  "nbformat": 4,