input: Write a python function to generate all permutations of a given string.
  assert generate_all_permutations("ABC")==["ABC", "ACB", "BAC", "BCA", "CBA", "CAB"]
  assert generate_all_permutations("AB")==["AB", "BA"]
  assert generate_all_permutations("A")==["A"]
output: def generate_all_permutations(s):
      if len(s) <= 1:
          return [s]
      
      permutations = []
      for i, char in enumerate(s):
          for perm in generate_all_permutations(s[:i] + s[i+1:]):
              permutations.append(char + perm)
      
      return permutations