input: Write a function to optimize a given multivariate function using the scipy library. The function should take a function and an initial guess as input, perform optimization, and return the optimized 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]**2 + x[1]**2 - x[0]*x[1], [2, 3])==[0.0, 0.0]
  assert optimize_function(lambda x: (x[0]-4)**2 + (x[1]-6)**2, [0, 0])==[4.0, 6.0]
output: def optimize_function(func, initial_guess):
      from scipy.optimize import minimize
      result = minimize(func, initial_guess)
      return result.x