Porting of 2D_diffusion_autoencoder tutorial to MONAI main from MONAI Generative (Project-MONAI#1871)

Porting of 2D diffusion autoencoder tutorial. 

### 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] 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
- [X] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`

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Signed-off-by: Virginia <virginia.fernandez@kcl.ac.uk>
Co-authored-by: Virginia <virginia.fernandez@kcl.ac.uk>

Author: virginiafdez