input: Write a python function to generate all permutations of a given string using itertools.
  assert permutations("abc")==[('a', 'b', 'c'), ('a', 'c', 'b'), ('b', 'a', 'c'), ('b', 'c', 'a'), ('c', 'a', 'b'), ('c', 'b', 'a')]
  assert permutations("ab")==[('a', 'b'), ('b', 'a')]
  assert permutations("a")==[('a',)]
  assert permutations("")==[]
output: def permutations(s):
      from itertools import permutations as perm
      return list(perm(s))