input: Write a python function to generate a random password of a given length using ascii letters and digits.
  assert len(generate_password(6))==6
  assert len(generate_password(10))==10
  assert len(generate_password(0))==0
output: import random
  import string
  
  def generate_password(length):
      characters = string.ascii_letters + string.digits
      return ''.join(random.choice(characters) for _ in range(length))