Adds empirical normalizer export when using rsl-rl workflow (isaac-sim#624)

Adds the empirical normalizer in the ONNX model export for RSL-RL `play.py`
script. Earlier, this was not being done, which would cause empirical
normalization to not work in the ONNX model.

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

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] 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
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

Author: pascal-roth

source/standalone/workflows/rsl_rl/play.py

@@ -79,12 +79,14 @@ def main():
     # obtain the trained policy for inference
     policy = ppo_runner.get_inference_policy(device=env.unwrapped.device)
 
-    # export policy to onnx
+    # export policy to onnx/jit
     export_model_dir = os.path.join(os.path.dirname(resume_path), "exported")
     export_policy_as_jit(
         ppo_runner.alg.actor_critic, ppo_runner.obs_normalizer, path=export_model_dir, filename="policy.pt"
     )
-    export_policy_as_onnx(ppo_runner.alg.actor_critic, path=export_model_dir, filename="policy.onnx")
+    export_policy_as_onnx(
+        ppo_runner.alg.actor_critic, normalizer=ppo_runner.obs_normalizer, path=export_model_dir, filename="policy.onnx"
+    )
 
     # reset environment
     obs, _ = env.get_observations()