6676 port generative networks ddpm (Project-MONAI#7304)

Towards Project-MONAI#6676  .

### Description

Adds a DDPM unet.

Refactoring for some of the blocks here is scheduled
[here](Project-MONAI#7227).

### 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).
- [x] 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`.
- [x] In-line docstrings updated.
- [x] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: Mark Graham <[email protected]>
Co-authored-by: YunLiu <[email protected]>
Signed-off-by: Mark Graham <[email protected]>

docs/source/networks.rst

@@ -583,6 +583,11 @@ Nets
 .. autoclass:: VNet
   :members:
 
+`DiffusionModelUnet`
+~~~~~~~~~~~~~~~~~~~~
+.. autoclass:: DiffusionModelUNet
+  :members:
+
 `RegUNet`
 ~~~~~~~~~
 .. autoclass:: RegUNet

monai/networks/nets/__init__.py

@@ -35,6 +35,7 @@
     densenet201,
     densenet264,
 )
+from .diffusion_model_unet import DiffusionModelUNet
 from .dints import DiNTS, TopologyConstruction, TopologyInstance, TopologySearch
 from .dynunet import DynUNet, DynUnet, Dynunet
 from .efficientnet import (