def __init__(self, names, fn):
        """
        Args:
            names (list[str]): names of tensors
            fn: a function taking all requested tensors as input
        """
        assert isinstance(names, (list, tuple)), names
        self._names = names
        self._fn = fn

        Args:
            names (list[str]): names of tensors
            fn: a function taking all requested tensors as input
        
tensorpack/callbacks/graph.py