input: Write a python function to extract the items and their prices from a nested dictionary and return them as a flat dictionary.
  assert extract_prices(menu) == {
   'Cheeseburger': 7,
   'Hamburger': 6,
   'Soda': 1,
   'Water': 1,
   'Mild': 10,
   'Hot': 12,
   'Ice Cream': 4
  }
  assert extract_prices({
   'Fruits': {'Apple': 3, 'Banana': 1},
   'Vegetables': {'Carrot': 2, 'Spinach': 5}
  }) == {'Apple': 3, 'Banana': 1, 'Carrot': 2, 'Spinach': 5}
  assert extract_prices({
   'Stationery': {'Pen': 2, 'Notebook': 5},
   'Electronics': {'Mouse': 10, 'Keyboard': 20}
  }) == {'Pen': 2, 'Notebook': 5, 'Mouse': 10, 'Keyboard': 20}
output: def extract_prices(menu):
      flat_dict = {}
      for category, items in menu.items():
          for item, price in items.items():
              flat_dict[item] = price
      return flat_dict