input: Write a python function to calculate the sum of elements in a nested list.
  assert calculate_sum([[1,2], [3,4], [6,7]]) == 23
  assert calculate_sum([[0,0], [0,0], [0,0]]) == 0
  assert calculate_sum([[5,5], [10,10], [15,15]]) == 60
output: def calculate_sum(nested_list):
      total = 0
      for sublist in nested_list:
          for item in sublist:
              total += item
      return total