input: Write a python function to add two matrices.
  assert add_matrix([[1,2],[3,4]], [[5,6],[7,8]]) == [[6,8],[10,12]]
  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],[4,5,6]], [[1,1,1],[1,1,1]]) == [[2,3,4],[5,6,7]]
output: def add_matrix(matrix1, matrix2):
      result = []
      for rows1, rows2 in zip(matrix1, matrix2):
          result.append([val1 + val2 for val1, val2 in zip(rows1, rows2)])
      return result