code: def download(self, dirpath):
          
          if os.path.isfile(self.lpath) and os.path.getsize(self.lpath) > 0:
              return
          print('Downloading %s -> %s' % (self.path, self.lpath))
          if dry_run:
              return
          self.arts.s3_bucket.download_file(self.path, self.lpath)
docs: Download artifact from S3 and store in dirpath directory.
              If the artifact is already downloaded nothing is done.