code: def get_offset(self):
          
          resp = requests.head(self.url, headers=self.headers)
          offset = resp.headers.get('upload-offset')
          if offset is None:
              msg = 'Attempt to retrieve offset fails with status {}'.format(resp.status_code)
              raise TusCommunicationError(msg, resp.status_code, resp.content)
          return int(offset)
docs: Return offset from tus server.
  
          This is different from the instance attribute 'offset' because this makes an
          http request to the tus server to retrieve the offset.