input: Write a function to find Armstrong numbers with `n` digits and return them as a list.
  assert find_armstrong_numbers(3) == [153, 370, 371, 407]
  assert find_armstrong_numbers(4) == [1634, 8208, 9474]
  assert find_armstrong_numbers(2) == []
output: def find_armstrong_numbers(n):
      def is_armstrong(num):
          digits = [int(d) for d in str(num)]
          return num == sum(d ** n for d in digits)
      
      start = 10**(n-1)
      end = 10**n
      return [num for num in range(start, end) if is_armstrong(num)]