fix warnings in back_prop

Author: HumphreyYang

lectures/back_prop.md

@@ -33,8 +33,9 @@ In addition to what's included in base Anaconda, we need to install the followin
 ```{code-cell} ipython3
 :tags: [hide-output]
 
-!pip install kaleido
-!conda install -y -c plotly plotly plotly-orca retrying
+import os
+os.system("pip install kaleido");
+os.system("conda install -y -c plotly plotly plotly-orca retrying");
 ```
 
 ```{note}