code: def step(self, action):
      x_position_before = self.sim.data.qpos[0]
      self.do_simulation(action, self.frame_skip)
      x_position_after = self.sim.data.qpos[0]
      x_velocity = (x_position_after - x_position_before) / self.dt
  
      ctrl_cost = self.control_cost(action)
  
      forward_reward = self._forward_reward_weight * x_velocity
  
      observation = self._get_obs()
      reward = forward_reward - ctrl_cost
      terminated = False
      info = {
          "x_position": x_position_after,
          "x_velocity": x_velocity,
          "reward_run": forward_reward,
          "reward_ctrl": -ctrl_cost,
      }
  
      if self.render_mode == "human":
          self.render()
      return observation, reward, terminated, False, info
  
file_path: gym/gym/envs/mujoco/half_cheetah_v3.py
imports: import numpy
  from gym import utils
  from gym.envs.mujoco import MuJocoPyEnv
  from gym.spaces import Box
label_desc: Use image node_id 3 for calling the HalfCheetahEnv obj's underlying member method code with example usage: obj.step(action) and returns: observation,  reward,  terminated,  False,  info