Add a progression bar for the current download.

def tqdm(self):
    """Add a progression bar for the current download."""
    async_tqdm = utils.async_tqdm
    with async_tqdm(total=0, desc='Dl Completed...', unit=' url') as pbar_url:
      with async_tqdm(total=0, desc='Dl Size...', unit=' MiB') as pbar_dl_size:
        self._pbar_url = pbar_url
        self._pbar_dl_size = pbar_dl_size
        yield