fix nan bug in hover_env due to numerical instability of angular dynamics (Genesis-Embodied-AI#1348)

Author: yun-long

examples/drone/hover_env.py

@@ -90,6 +90,9 @@ def __init__(self, num_envs, env_cfg, obs_cfg, reward_cfg, command_cfg, show_vie
         # build scene
         self.scene.build(n_envs=num_envs)
 
+        # Set damping to a small value to avoid numerical instability
+        self.drone.set_dofs_damping(torch.tensor([0.0, 0.0, 0.0, 1e-4, 1e-4, 1e-4]))
+
         # prepare reward functions and multiply reward scales by dt
         self.reward_functions, self.episode_sums = dict(), dict()
         for name in self.reward_scales.keys():