Add Visualization in Controlnet Model Training Tutorial (#1861)

Fixes #1855

### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [ ] Avoid including large-size files in the PR.
- [ ] Clean up long text outputs from code cells in the notebook.
- [ ] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [ ] Ensure (1) hyperlinks and markdown anchors are working (2) use
relative paths for tutorial repo files (3) put figure and graphs in the
`./figure` folder
- [ ] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`

---------

Signed-off-by: YunLiu <55491388+KumoLiu@users.noreply.github.com>

Author: KumoLiu

generation/maisi/figures/maisi_control-train-examples.png

@@ -40,7 +40,8 @@
     "For additional information about VISTA-2D please also refer the [MONAI bundle](https://github.com/Project-MONAI/VISTA/tree/main/vista2d) and the [technical blog post](https://developer.nvidia.com/blog/advancing-cell-segmentation-and-morphology-analysis-with-nvidia-ai-foundation-model-vista-2d/).\n",
     "\n",
     "For dependency-related issues, you can also check this note:\n",
-    "https://github.com/Project-MONAI/model-zoo/tree/dev/models/vista2d#dependencies."
+    "https://github.com/Project-MONAI/model-zoo/tree/dev/models/vista2d#dependencies.\n",
+    "If you're using MONAI's container, you can disregard the 'opencv-python-headless not installed' error, as it is already included within the container."
    ]
   },
   {