input: Write a python function to add two matrices using numpy.
  assert np.array_equal(add_matrices([[1, 2, 3], [4, 5, 6]], [[7, 8, 9], [10, 11, 12]]), [[8, 10, 12], [14, 16, 18]])
  assert np.array_equal(add_matrices([[1, 2], [3, 4]], [[5, 6], [7, 8]]), [[6, 8], [10, 12]])
  assert np.array_equal(add_matrices([[0, 0], [0, 0]], [[0, 0], [0, 0]]), [[0, 0], [0, 0]])
output: import numpy as np
  
  def add_matrices(mat1, mat2):
      return np.add(mat1, mat2).tolist()