Update networks doc (#6671)

Fixes #6670.

### Description

Add `SegResNetDS` in the doc

### 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.

---------

Signed-off-by: KumoLiu <yunl@nvidia.com>

docs/source/networks.rst

@@ -471,6 +471,11 @@ Nets
 .. autoclass:: SegResNet
   :members:
 
+`SegResNetDS`
+~~~~~~~~~~~~~
+.. autoclass:: SegResNetDS
+  :members:
+
 `SegResNetVAE`
 ~~~~~~~~~~~~~~
 .. autoclass:: SegResNetVAE