input: Write a function to add two numpy arrays element-wise.
  assert np.array_equal(add_vector(np.array([1, 2, 3]), np.array([4, 5, 6])), np.array([5, 7, 9]))
  assert np.array_equal(add_vector(np.array([10, 20]), np.array([30, 40])), np.array([40, 60]))
  assert np.array_equal(add_vector(np.array([0, 0, 0]), np.array([1, 1, 1])), np.array([1, 1, 1]))
output: import numpy as np
  
  def add_vector(a, b):
      return np.add(a, b)