input: Write a python function to return the Fibonacci number at a given index.
  assert fibonacci_at_index(5) == 5
  assert fibonacci_at_index(10) == 55
  assert fibonacci_at_index(0) == 0
  assert fibonacci_at_index(1) == 1
  assert fibonacci_at_index(7) == 13
output: def fibonacci_at_index(index):
      if index == 0:
          return 0
      elif index == 1:
          return 1
      a, b = 0, 1
      for _ in range(2, index + 1):
          a, b = b, a + b
      return b