input: Write a python function to generate a unique 5-digit ID using random integers.
  assert len(generate_id()) == 5
  assert generate_id().isdigit()
  assert len(set([generate_id() for _ in range(1000)])) > 900
output: import random
  
  def generate_id():
      return ''.join(str(random.randint(0, 9)) for _ in range(5))