Return the base URL for Gluon dataset and model repository.

def _get_repo_url():
    """Return the base URL for Gluon dataset and model repository."""
    default_repo = 'https://apache-mxnet.s3-accelerate.dualstack.amazonaws.com/'
    repo_url = os.environ.get('MXNET_GLUON_REPO', default_repo)
    if repo_url[-1] != '/':
        repo_url = repo_url+'/'
    return repo_url