Parses command line arguments.

def parse_args():
  """Parses command line arguments."""
  parser = argparse.ArgumentParser(
      description='Tool to download dataset images.')
  parser.add_argument('--input_file', required=True,
                      help='Location of dataset.csv')
  parser.add_argument('--output_dir', required=True,
                      help='Output path to download images')
  parser.add_argument('--threads', default=multiprocessing.cpu_count() + 1,
                      help='Number of threads to use')
  args = parser.parse_args()
  return args.input_file, args.output_dir, int(args.threads)