input: Write a python function to determine if two numbers have exactly one bit difference.
  assert differ_At_One_Bit(13, 9) == True
  assert differ_At_One_Bit(15, 8) == False
  assert differ_At_One_Bit(2, 4) == False
output: def differ_At_One_Bit(a, b):
      x = a ^ b
      return x != 0 and (x & (x - 1)) == 0