6676 port generative networks autoencoderkl (#7260)

Partially fixes #6676 


### Description
Implements the AutoencoderKL network from MONAI Generative.

NB this network is subject to a planned refactor once the porting is
complete, [see
here](#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.
- [x] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [x] 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: Eric Kerfoot <[email protected]>
Co-authored-by: YunLiu <[email protected]>

docs/source/networks.rst

@@ -595,6 +595,11 @@ Nets
 .. autoclass:: AutoEncoder
   :members:
 
+`AutoEncoderKL`
+~~~~~~~~~~~~~~~
+.. autoclass:: AutoencoderKL
+  :members:
+
 `VarAutoEncoder`
 ~~~~~~~~~~~~~~~~
 .. autoclass:: VarAutoEncoder

monai/networks/nets/__init__.py

@@ -14,6 +14,7 @@
 from .ahnet import AHnet, Ahnet, AHNet
 from .attentionunet import AttentionUnet
 from .autoencoder import AutoEncoder
+from .autoencoderkl import AutoencoderKL
 from .basic_unet import BasicUNet, BasicUnet, Basicunet, basicunet
 from .basic_unetplusplus import BasicUNetPlusPlus, BasicUnetPlusPlus, BasicunetPlusPlus, basicunetplusplus
 from .classifier import Classifier, Critic, Discriminator