Fixes unwanted squeeze in Articulation class for 1-joint assets (#397)

# Description

This MR fixes the issue with printing articulation joint information
with the `Articulation` class. When the number of joints are 1, the
`squeeze(0).tolist()` operation makes the tensor a float item instead of
a list.

This MR removes the squeeze operation and adds a unit test to check that
single-joint articulations work.

Fixes https://github.com/NVIDIA-Omniverse/orbit/issues/238

## 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`
- [x] 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 run all the tests with `./orbit.sh --test` and they pass
- [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 = "0.10.20"
+version = "0.10.21"
 
 # Description
 title = "ORBIT framework for Robot Learning"

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

 Changelog
 ---------
 
+0.10.21 (2024-02-12)
+~~~~~~~~~~~~~~~~~~~~
+
+Fixed
+^^^^^
+
+* Fixed the printing of articulation joint information when the articulation has only one joint.
+  Earlier, the function was performing a squeeze operation on the tensor, which caused an error when
+  trying to index the tensor of shape (1,).
+
+
 0.10.20 (2024-02-12)
 ~~~~~~~~~~~~~~~~~~~~
 

source/extensions/omni.isaac.orbit/omni/isaac/orbit/assets/articulation/articulation.py

@@ -703,15 +703,15 @@ class Articulation(RigidObject):
         """Log information about the articulation's simulated joints."""
         # read out all joint parameters from simulation
         # -- gains
-        stiffnesses = self.root_physx_view.get_dof_stiffnesses()[0].squeeze(0).tolist()
-        dampings = self.root_physx_view.get_dof_dampings()[0].squeeze(0).tolist()
+        stiffnesses = self.root_physx_view.get_dof_stiffnesses()[0].tolist()
+        dampings = self.root_physx_view.get_dof_dampings()[0].tolist()
         # -- properties
-        armatures = self.root_physx_view.get_dof_armatures()[0].squeeze(0).tolist()
-        frictions = self.root_physx_view.get_dof_friction_coefficients()[0].squeeze(0).tolist()
+        armatures = self.root_physx_view.get_dof_armatures()[0].tolist()
+        frictions = self.root_physx_view.get_dof_friction_coefficients()[0].tolist()
         # -- limits
-        position_limits = self.root_physx_view.get_dof_limits()[0].squeeze(0).tolist()
-        velocity_limits = self.root_physx_view.get_dof_max_velocities()[0].squeeze(0).tolist()
-        effort_limits = self.root_physx_view.get_dof_max_forces()[0].squeeze(0).tolist()
+        position_limits = self.root_physx_view.get_dof_limits()[0].tolist()
+        velocity_limits = self.root_physx_view.get_dof_max_velocities()[0].tolist()
+        effort_limits = self.root_physx_view.get_dof_max_forces()[0].tolist()
         # create table for term information
         table = PrettyTable(float_format=".3f")
         table.title = f"Simulation Joint Information (Prim path: {self.cfg.prim_path})"

source/extensions/omni.isaac.orbit/test/assets/test_articulation.py

@@ -147,6 +147,44 @@ class TestArticulation(unittest.TestCase):
             # update robot
             robot.update(self.dt)
 
+    def test_initialization_fixed_base_single_joint(self):
+        """Test initialization for fixed base articulation with a single joint."""
+        # Create articulation
+        robot_cfg = ArticulationCfg(
+            spawn=sim_utils.UsdFileCfg(usd_path=f"{ISAAC_NUCLEUS_DIR}/Robots/Simple/revolute_articulation.usd"),
+            actuators={
+                "joint": ImplicitActuatorCfg(
+                    joint_names_expr=[".*"],
+                    effort_limit=400.0,
+                    velocity_limit=100.0,
+                    stiffness=0.0,
+                    damping=10.0,
+                ),
+            },
+        )
+        robot = Articulation(cfg=robot_cfg.replace(prim_path="/World/Robot"))
+
+        # Check that boundedness of articulation is correct
+        self.assertEqual(ctypes.c_long.from_address(id(robot)).value, 1)
+
+        # Play sim
+        self.sim.reset()
+        # Check if robot is initialized
+        self.assertTrue(robot._is_initialized)
+        # Check that fixed base
+        self.assertTrue(robot.is_fixed_base)
+        # Check buffers that exists and have correct shapes
+        self.assertTrue(robot.data.root_pos_w.shape == (1, 3))
+        self.assertTrue(robot.data.root_quat_w.shape == (1, 4))
+        self.assertTrue(robot.data.joint_pos.shape == (1, 1))
+
+        # Simulate physics
+        for _ in range(10):
+            # perform rendering
+            self.sim.step()
+            # update robot
+            robot.update(self.dt)
+
     def test_external_force_on_single_body(self):
         """Test application of external force on the base of the robot."""