[BugFix] _set_obj_xyz in stick-push/stick-pull uses wrong qvel indices, causing object to drift after reset

[15101051]

In SawyerStickPushEnvV3 and SawyerStickPullEnvV3, the _set_obj_xyz method zeroes qvel[16:18] to clear the object's velocity. However, due to the stick body using a free joint (7 qpos but only 6 qvel components), there is a +1 offset between qpos and qvel indices for all bodies defined after the stick.
The object's slide joint velocities are actually at qvel[15:17], not qvel[16:18].

  # Current (buggy):
  def _set_obj_xyz(self, pos):
      qpos = self.data.qpos.flat.copy()
      qvel = self.data.qvel.flat.copy()
      qpos[16:18] = pos.copy()
      qvel[16:18] = 0          # ← wrong: zeroes qvel[16] and qvel[17]
      self.set_state(qpos, qvel)

  # Fixed:
      qvel[15:17] = 0          # ← correct: zeroes the actual slide joint velocities

Observed behavior: After reset(), the thermos retains a nonzero x-velocity (~0.68) from the _reset_hand() simulation steps. It slides along +x until hitting the joint limit (offset = 0.2, world x ≈ 0.4), which coincidentally aligns with the target position — making it appear as if the object "teleports" to the goal.

Affected files:

• metaworld/envs/sawyer_stick_push_v3.py
• metaworld/envs/sawyer_stick_pull_v3.py