MujocoEnv reset_mujoco doesn't work

[henry-zhang-bohan]

When calling reset() on MujocoEnv, it gives the following error:

Traceback (most recent call last):
  File "garage/scripts/run_experiment.py", line 242, in <module>
    run_experiment(sys.argv)
  File "garage/scripts/run_experiment.py", line 185, in run_experiment
    method_call(variant_data)
  File "./examples/pusher_pretrain.py", line 81, in run_experiment
    env = normalize(PusherEnv(goal=variant.get('goal')))
  File "softqlearning/softqlearning/environments/pusher.py", line 37, in __init__
    super(PusherEnv, self).__init__(file_path=self.FILE_PATH)
  File "garage/garage/envs/mujoco/mujoco_env.py", line 85, in __init__
    self.reset()
  File "softqlearning/softqlearning/environments/pusher.py", line 130, in reset
    super(PusherEnv, self).reset(full_state)
  File "garage/garage/envs/mujoco/mujoco_env.py", line 134, in reset
    self.reset_mujoco(init_state)
  File "garage/garage/envs/mujoco/mujoco_env.py", line 122, in reset_mujoco
    self.sim.data.qpos = datum
AttributeError: attribute 'qpos' of 'mujoco_py.cymj.PyMjData' objects is not writable

It seems like a mujoco-py=1.50 issue: openai/mujoco-py#266