Fix to channel specification in 3d_autoencoderkl notebook (#1925)

Fixes #1924 

Modification of channel definition within 3d_autoencoderkl notebook. 

### Description
The variable "channel" was wrongly defined after the transforms.
I have re-run the plot for channel = 2 and it runs fine. 

### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [X] Avoid including large-size files in the PR.
- [X] Clean up long text outputs from code cells in the notebook.
- [X] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [X] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`

---------

Signed-off-by: Virginia <virginia.fernandez@kcl.ac.uk>
Signed-off-by: Virginia Fernandez <virginia.fernandez@kcl.ac.uk>
Co-authored-by: Virginia <virginia.fernandez@kcl.ac.uk>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

generation/3d_autoencoderkl/3d_autoencoderkl_tutorial.ipynb

@@ -178,6 +178,8 @@
    "metadata": {},
    "outputs": [],
    "source": [
+    "channel = 0  # 0 = Flair\n",
+    "assert channel in [0, 1, 2, 3], \"Choose a valid channel\"\n",
     "all_transforms = [\n",
     "    transforms.LoadImaged(keys=[\"image\"]),\n",
     "    transforms.EnsureChannelFirstd(keys=[\"image\"]),\n",
@@ -207,8 +209,6 @@
     }
    ],
    "source": [
-    "channel = 0  # 0 = Flair\n",
-    "assert channel in [0, 1, 2, 3], \"Choose a valid channel\"\n",
     "# Training\n",
     "train_ds = DecathlonDataset(\n",
     "    root_dir=root_dir,\n",
@@ -785,7 +785,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.12.0"
+   "version": "3.10.15"
   }
  },
  "nbformat": 4,