Fix to channel specification in 3d_autoencoderkl notebook (#1925)

Fixes #1924 

Modification of channel definition within 3d_autoencoderkl notebook. 

### Description
The variable "channel" was wrongly defined after the transforms.
I have re-run the plot for channel = 2 and it runs fine. 

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

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Signed-off-by: Virginia <virginia.fernandez@kcl.ac.uk>
Signed-off-by: Virginia Fernandez <virginia.fernandez@kcl.ac.uk>
Co-authored-by: Virginia <virginia.fernandez@kcl.ac.uk>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>