DDPM Tutorials (#1814)

Address some of #1769.

### Description
These are the 2D ~~and 3D~~ DDPM tutorials. I have run them again to
verify they work. ~~The 2D ones are fine but the 3D tutorial keeps
having mode collapse or some other issue resulting in bad results. Work
is ongoing on this one.~~ I have left the 3D tutorial off since it's not
training stably and needs some revamp to be a proper and useful tutorial
for users.

### 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>`

---------

Signed-off-by: Eric Kerfoot <eric.kerfoot@kcl.ac.uk>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: YunLiu <55491388+KumoLiu@users.noreply.github.com>