Create an `ItemList` in `path` from the inputs in the `cols` of `df`.

def from_df(cls, df:DataFrame, path:PathOrStr='.', cols:IntsOrStrs=0, processor:PreProcessors=None, **kwargs)->'ItemList':
        "Create an `ItemList` in `path` from the inputs in the `cols` of `df`."
        inputs = df.iloc[:,df_names_to_idx(cols, df)]
        assert inputs.isna().sum().sum() == 0, f"You have NaN values in column(s) {cols} of your dataframe, please fix it."
        res = cls(items=_maybe_squeeze(inputs.values), path=path, inner_df=df, processor=processor, **kwargs)
        return res