Fixes None device in AppLauncher CLI (#300)

# Description

A recent change changed the default behavior of CLI device parsing in
AppLauncher, where if None was passed to the CLI, the CLI device
argument stayed as None instead of setting to the default cuda:0 device.
This change reverts to the previous behavior and restores cuda:0 as the
default device if None is passed, unless --xr is also passed, in which
case, we default to cpu.

## Type of change

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applicable. -->

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

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source/isaaclab/isaaclab/app/app_launcher.py

@@ -257,6 +257,7 @@ class AppLauncher:
         arg_group.add_argument(
             "--device",
             type=str,
+            default=AppLauncher._APPLAUNCHER_CFG_INFO["device"][1] if "--xr" not in sys.argv else None,
             help='The device to run the simulation on. Can be "cpu", "cuda", "cuda:N", where N is the device ID',
         )
         # Add the deprecated cpu flag to raise an error if it is used