input: Write a python function to find all Armstrong numbers in the range from 1 to 20.
  assert find_armstrong_numbers(1, 20) == [1]
  assert find_armstrong_numbers(0, 100) == [0, 1]
  assert find_armstrong_numbers(10, 1000) == [153, 370, 371, 407]
output: def find_armstrong_numbers(start, end):
      def is_armstrong(n):
          num_str = str(n)
          num_len = len(num_str)
          return n == sum(int(digit) ** num_len for digit in num_str)
  
      return [num for num in range(start, end) if is_armstrong(num)]