input: Write a function to optimize a given multivariate function using the `scipy.optimize` module. This function should take in the function to be optimized and an initial guess for the optimization process, and return the optimal values of the variables.
  
  assert optimize_function(lambda x: x[0]**2 + x[1]**2, [1, 1])==[0.0, 0.0]
  assert optimize_function(lambda x: (x[0]-1)**2 + (x[1]+1)**2, [0, 0])==[1.0, -1.0]
output: def optimize_function(func, initial_guess):
      from scipy.optimize import minimize
      result = minimize(func, initial_guess)
      return result.x