Adds missing robot imports (#3976)

# Description

Adds a couple of robots that were missing from the __init__.py imports

Fixes #3881 

## Type of change

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

## Checklist

- [x] I have read and understood the [contribution
guidelines](https://isaac-sim.github.io/IsaacLab/main/source/refs/contributing.html)
- [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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Author: kellyguo11

source/isaaclab_assets/isaaclab_assets/robots/__init__.py

@@ -7,11 +7,14 @@
 # Configuration for different assets.
 ##
 
+from .agibot import *
+from .agility import *
 from .allegro import *
 from .ant import *
 from .anymal import *
 from .cart_double_pendulum import *
 from .cartpole import *
+from .cassie import *
 from .fourier import *
 from .franka import *
 from .galbot import *