input: Write a function to add two matrices.
  assert add_matrix([[1,2], [3,4]], [[5,6,7], [8,9,10], [1,4,6]]) == [[6, 8], [11, 13]]
  assert add_matrix([[1,2,3], [4,5,6]], [[7,8,9], [10,11,12]]) == [[8, 10, 12], [14, 16, 18]]
  assert add_matrix([[1]], [[2]]) == [[3]]
output: def add_matrix(matrix1, matrix2):
      return [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]