input: Write a python function to initialize a dictionary with keys from 0 to size-1 and values set to None.
  assert initialize_dict(5) == {0: None, 1: None, 2: None, 3: None, 4: None}
  assert initialize_dict(3) == {0: None, 1: None, 2: None}
  assert initialize_dict(0) == {}
output: def initialize_dict(size):
      return {i: None for i in range(size)}