input: Write a python function to compute the cumulative sum of each row in a matrix.
  assert accumulator_add([[1, 2, 3], [4, 5, 6]]) == [[1, 3, 6], [4, 9, 15]]
  assert accumulator_add([[0, 0, 0], [1, 2, 3]]) == [[0, 0, 0], [1, 3, 6]]
  assert accumulator_add([[1], [2], [3]]) == [[1], [2], [3]]
output: def accumulator_add(matrix):
      result = []
      for row in matrix:
          cumulative_sum = 0
          new_row = []
          for num in row:
              cumulative_sum += num
              new_row.append(cumulative_sum)
          result.append(new_row)
      return result