Implemented VoxelMorph (#7178)

Fixes #5484.

### Description

Implemented VoxelMorph and added some docstrings. Checked coding style
locally.

### 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: kaibo <ktang@unc.edu>

docs/source/networks.rst

@@ -710,6 +710,18 @@ Nets
 .. autoclass:: Quicknat
  :members:
 
+`VoxelMorph`
+~~~~~~~~~~~~
+.. autoclass:: VoxelMorphUNet
+ :members:
+
+.. autoclass:: voxelmorphunet
+ :members:
+
+.. autoclass:: VoxelMorph
+ :members:
+
+.. autoclass:: voxelmorph
 
 Utilities
 ---------

monai/networks/nets/__init__.py

@@ -111,3 +111,4 @@
 from .vit import ViT
 from .vitautoenc import ViTAutoEnc
 from .vnet import VNet
+from .voxelmorph import VoxelMorph, VoxelMorphUNet