Integrating a Tailored Diffusion U-Net Model into Generative Model Ap…

…plication (Project-MONAI#7867)

Fixes Project-MONAI#7858.

Integrating a tailored diffusion U-Net model into the generative model
application to improve the production of high-dimensional 3D images,
specifically sized at 512 x 512 x 512.

A few sentences describing the changes proposed in this pull request.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

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Signed-off-by: dongyang0122 <don.yang.mech@gmail.com>
Signed-off-by: Dong Yang <yangd05@Dongs-MacBook-Pro.local>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Dong Yang <yangd05@Dongs-MacBook-Pro.local>
Co-authored-by: YunLiu <55491388+KumoLiu@users.noreply.github.com>