Fix to install torch and torchvision on 1_VAEs.ipynb

Fix to install torch and torchvision on `1_VAEs.ipynb`

I had to do the following in the notebook (running on github codespaces)

https://github.com/acceleratescience/diffusion-models/blob/main/notebooks/1_VAEs.ipynb


```py
pip install torch torchvision
```