input: Write a python function to parse datetime from a string, where the string might be in the form "yyyy-mm-dd HH:MM" or "HH:MM".
  assert parse_datetime("2023-10-01 12:30") == datetime(2023, 10, 1, 12, 30)
  assert parse_datetime("14:45").hour == 14
  assert parse_datetime("14:45").minute == 45
output: def parse_datetime(datetime_str):
      from datetime import datetime
      
      if " " in datetime_str:
          return datetime.strptime(datetime_str, "%Y-%m-%d %H:%M")
      else:
          today = datetime.today()
          time_part = datetime.strptime(datetime_str, "%H:%M").time()
          return datetime.combine(today, time_part)