code: def brownian_motion_unknown_scales_missing_middle_observations():
      """Brownian Motion with missing observations and unknown scale parameters.
  
      Returns:
        target: StanModel.
      """
      dataset = data.brownian_motion_missing_middle_observations()
      return brownian_motion.brownian_motion_unknown_scales(
          observed_locs=dataset["observed_locs"]
      )
  
imports: from inference_gym.internal import data
  from inference_gym.tools.stan import brownian_motion
  from inference_gym.tools.stan import eight_schools
  from inference_gym.tools.stan import item_response_theory
  from inference_gym.tools.stan import log_gaussian_cox_process
  from inference_gym.tools.stan import logistic_regression
  from inference_gym.tools.stan import lorenz_system
  from inference_gym.tools.stan import probit_regression
  from inference_gym.tools.stan import radon_contextual_effects
  from inference_gym.tools.stan import radon_contextual_effects_halfnormal
  from inference_gym.tools.stan import sparse_logistic_regression
  from inference_gym.tools.stan import stochastic_volatility
label_desc: Use image node_id 2 for calling a global function with example usage: brownian_motion_unknown_scales_missing_middle_observations() and returns: brownian_motion