Fixes the RuntimeError when using null command generator (#230)

# Description

The null-command generator class sets the resampling time as
`(-math.inf, math.inf)`. However, on execution, the random ampler does
not like this range and throws a RuntimeError as the operation becomes
out of bounds for float.

To deal with this issue, the MR just overrides all the functions of the
base class and leaves them "empty".

## Type of change

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

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] 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

source/extensions/omni.isaac.orbit/config/extension.toml

 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "0.9.32"
+version = "0.9.33"
 
 # Description
 title = "ORBIT framework for Robot Learning"

source/extensions/omni.isaac.orbit/docs/CHANGELOG.rst

 Changelog
 ---------
 
+0.9.33 (2023-11-02)
+~~~~~~~~~~~~~~~~~~~
+
+Fixed
+^^^^^
+
+* Fixed the :class:`omni.isaac.orbit.command_generators.NullCommandGenerator` class. Earlier,
+  it was having a runtime error due to infinity in the resampling time range. Now, the class just
+  overrides the parent methods to perform no operations.
+
+
 0.9.32 (2023-11-02)
 ~~~~~~~~~~~~~~~~~~~
 

source/extensions/omni.isaac.orbit/omni/isaac/orbit/command_generators/null_command_generator.py

@@ -25,6 +25,12 @@ class NullCommandGenerator(CommandGeneratorBase):
     cfg: NullCommandGeneratorCfg
     """Configuration for the command generator."""
 
+    def __str__(self) -> str:
+        msg = "NullCommandGenerator:\n"
+        msg += "\tCommand dimension: N/A\n"
+        msg += f"\tResampling time range: {self.cfg.resampling_time_range}"
+        return msg
+
     """
     Properties
     """
@@ -38,6 +44,16 @@ class NullCommandGenerator(CommandGeneratorBase):
         """
         raise RuntimeError("NullCommandGenerator does not generate any commands.")
 
+    """
+    Operations.
+    """
+
+    def reset(self, env_ids: Sequence[int] | None = None) -> dict[str, float]:
+        return {}
+
+    def compute(self, dt: float):
+        pass
+
     """
     Implementation specific functions.
     """

source/extensions/omni.isaac.orbit/test/command_generators/test_null_command_generator.py

+# Copyright (c) 2022-2023, The ORBIT Project Developers.
+# All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+
+from __future__ import annotations
+
+"""Launch Isaac Sim Simulator first."""
+
+from omni.isaac.kit import SimulationApp
+
+# launch omniverse app
+config = {"headless": True}
+simulation_app = SimulationApp(config)
+
+"""Rest everything follows."""
+
+import unittest
+from collections import namedtuple
+
+from omni.isaac.orbit.command_generators import NullCommandGeneratorCfg
+
+
+class TestNullCommandGeneratorCfg(unittest.TestCase):
+    """Test cases for null command generator."""
+
+    def setUp(self) -> None:
+        self.env = namedtuple("RLTaskEnv", ["num_envs", "dt", "device"])(20, 0.1, "cpu")
+
+    def test_str(self):
+        """Test the string representation of the command manager."""
+        cfg = NullCommandGeneratorCfg()
+        command_manager = cfg.class_type(cfg, self.env)
+        # print the expected string
+        print()
+        print(command_manager)
+
+    def test_compute(self):
+        """Test the compute function. For null command generator, it does nothing."""
+        cfg = NullCommandGeneratorCfg()
+        command_manager = cfg.class_type(cfg, self.env)
+
+        # test the reset function
+        command_manager.reset()
+        # test the compute function
+        command_manager.compute(dt=self.env.dt)
+        # expect error
+        with self.assertRaises(RuntimeError):
+            command_manager.command
+
+
+if __name__ == "__main__":
+    unittest.main()