input: Write a function to create a Python generator that returns numbers from a given list. If the given list is empty then the generator should yield nothing.
  assert list(list_generator([1,3,5,7,9]))==[1,3,5,7,9]
  assert list(list_generator([]))==[]
  assert list(list_generator([2]))==[2]
output: def list_generator(lst):
      for num in lst:
          yield num