Addition of classifier-free guidance notebook (#1867)

### Description
Addition of classifier-free guidance notebook for the use of CFG on
DDPMs.

### Checks
- [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 Fernandez <virginia.fernandez@kcl.ac.uk>
Signed-off-by: Eric Kerfoot <eric.kerfoot@kcl.ac.uk>
Signed-off-by: Virginia <virginia.fernandez@kcl.ac.uk>
Co-authored-by: Virginia Fernandez <virginia.fernandez@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>
Co-authored-by: Eric Kerfoot <eric.kerfoot@kcl.ac.uk>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>