input: Write a python function to convert polar coordinates to cartesian coordinates.
  assert polar_to_cartesian(1, 0) == (1.0, 0.0)
  assert polar_to_cartesian(1, np.pi/2) == (0.0, 1.0)
  assert polar_to_cartesian(2, np.pi) == (-2.0, 0.0)
output: import numpy as np
  
  def polar_to_cartesian(r, theta):
      x = r * np.cos(theta)
      y = r * np.sin(theta)
      return (x, y)